What If McClellan isn't Removed in Fall of 1862?

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Saphroneth

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It's hilarious to zoom in on those routes. Did they really take all those little detours? I doubt it.
I fear I should point out that we don't really have any better way to measure the over-road distance marched than using a source like Google Maps. The source does add "little detours" (which usually means moving down roads that were not straight - the Urbana Pike is not straight and is a historical road - or filtering through the streets of a town) but if we use Google Maps consistently for all measurements of distance we can at least presume that most errors would be in the same direction. On the other hand, using the straight-line distance from start point to end point will inevitably produce a lower estimate, because even if the straight-line road exists it would only admit ca. 30,000 troops per day - this is how many troops can fit down a road in one day.

If all of McClellan's army took the shortest straight-line route from a camp at Tenleytown to the middle of Frederick, then the length of the route would be about forty miles; however, it would take three full days to fit his army down this road. This means that at a march rate of ten miles per march his army would start marching on the 9th (say), his vanguard would march on the 9th-12th inclusive and his rearguard would end their march on the end of the 15th (being thirty miles behind) - for an apparent rate of march of only ~6 miles per day.
Moving in columns along parallel roads increases march distance but reduces "traffic jams" and this is why armies did it; a representation of march distance that does not include this is inaccurate. In fact, if desired I can examine a specific scenario of the ones I looked at above which does consider marching along parallel roads, and this would likely slow McClellan's army down somewhat.

This is why when looking at the distance Jackson could consider his troops to be able to march in a day based on recent experience, above (where I cite the movement from Harpers Ferry to Boteler's Ford) I used the modern road network and caveated it as such; if Jackson was actually able to take a more straight route with less in the way of the little detours forced by the modern road network, his estimated march speed before straggling to bits would be lower.
 

Dead Parrott

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I fear I should point out that we don't really have any better way to measure the over-road distance marched than using a source like Google Maps. The source does add "little detours" (which usually means moving down roads that were not straight - the Urbana Pike is not straight and is a historical road - or filtering through the streets of a town) but if we use Google Maps consistently for all measurements of distance we can at least presume that most errors would be in the same direction. On the other hand, using the straight-line distance from start point to end point will inevitably produce a lower estimate, because even if the straight-line road exists it would only admit ca. 30,000 troops per day - this is how many troops can fit down a road in one day.

If all of McClellan's army took the shortest straight-line route from a camp at Tenleytown to the middle of Frederick, then the length of the route would be about forty miles; however, it would take three full days to fit his army down this road. This means that at a march rate of ten miles per march his army would start marching on the 9th (say), his vanguard would march on the 9th-12th inclusive and his rearguard would end their march on the end of the 15th (being thirty miles behind) - for an apparent rate of march of only ~6 miles per day.
Moving in columns along parallel roads increases march distance but reduces "traffic jams" and this is why armies did it; a representation of march distance that does not include this is inaccurate. In fact, if desired I can examine a specific scenario of the ones I looked at above which does consider marching along parallel roads, and this would likely slow McClellan's army down somewhat.

This is why when looking at the distance Jackson could consider his troops to be able to march in a day based on recent experience, above (where I cite the movement from Harpers Ferry to Boteler's Ford) I used the modern road network and caveated it as such; if Jackson was actually able to take a more straight route with less in the way of the little detours forced by the modern road network, his estimated march speed before straggling to bits would be lower.
A very difficult exercise to do, I know. Years ago I did a similar analysis of Marlborough's campaign movements, as part of a research paper. Even with contemporary records (establishing depots, etc.), there's considerable variance in movement rates and army disorganization, for a wide variety of reasons - even on paths travelled frequently before.

It's It can be an exhausting and rewarding compilation effort - and it teaches you to be very suspect of 'assured' capabilities of movement rates and army readiness in any given moment.

Sometimes those variables don't 'collapse upon the Mean' before the army itself collapses!
 

Saphroneth

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It's It can be an exhausting and rewarding compilation effort - and it teaches you to be very suspect of 'assured' capabilities of movement rates and army readiness in any given moment.
Oh, indeed; it's one reason why a lot of what I've been looking at is best estimates (that is, using a fairly consistent movement speed of ca. 9-11 miles per day of movement) and deliberately nudging that value higher for the CS units and lower for the US ones. I know that I am more inclined to see the US succeed in this situation so I am giving the CS the advantage in movement speed even when both sides seem functionally identical.

It looks like the main theoretical cause of congestion is the approaches to Culpeper itself. The OR atlas map gives one road coming down on Culpeper from the north (the route from Warrenton passes through Jefferson) which is fairly direct, and the next route to the west involves going through Fayetteville and then marching a few miles perpendicular to the direction of Fayetteville before moving on the town from east of northeast. That's two lines of march which combine there, or the forces marching from east of northeast could turn off and go via Stevensburg to Raccoon Ford (and thus avoid congesting the line of march of the first force going Warrenton-Jefferson-Culpeper-Rapidan).
If McClellan wanted instead to send a force swinging west of Culpeper without passing through it he'd have to go out as far as Woodville or Sperryville, which would mean a much longer march; as they say, the devil's in the details and this is getting quite detailed. If I was to assume a homogenous movement speed of ~9 miles per day for the Union forces and ~10 miles per day for the Confederate forces - as averages - then I could probably look at it that way.

parallel_march_routes.jpg

The closest estimate Google Maps gives me is that the western route is between 37 and 39 miles long - functionally four days marching with the Rapidan crossing happening on the fifth if there's any opposition there. The eastern route seems to be about 31 miles long before reaching the Rapidan, so the crossing there would happen early on the fourth day.
Note that if Culpeper is defended in strength then McClellan's eastern column (or part of it) can simply not turn off the road, converging on the town; this is part of how he can compel Longstreet to abandon the town. If Longstreet's whole strength is defending the line of the river running through Culpeper then McClellan can cross his eastern column at Stevensburg and cut west to turn the position.


(Once McClellan has his forces on the Orange turnpike to Fredericksburg, he can just head straight down that - using a single march route in that case isn't as much of a problem. But having parallel marching routes in the area between the Rappahanock and the Rapidan both saves at least a day and also makes Longstreet's task of holding the line of the Rapidan much harder.)
 
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67th Tigers

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What is certain about Jackson is that when McClellan sent the order to abandon Snicker's Gap on 6th November at 2200 4th November, Jackson no longer posed any threat to McClellan's LOC. McClellan set up a depot at Gainesville, guarded by 11th Corps and Sickles' Division. This is about 15 miles from Warrenton, and the wagons were refilling from there.
 

Saphroneth

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Speaking of wagons, one important constraint is the amount of food that could be supplied to the army before the next move between supply sources.
My suspicion is that it could be reasonably modelled that the army had full wagons at Warrenton (because the army was able to sustain itself there for more than a week after McClellan's relief and then march to Fredericksburg north of the Rappahanock) but that once the army moves south of Warrenton Junction the section of line between Warrenton Junction via Culpeper to Orange should be modelled as having only the capacity for ca. 50,000 man-days of food per day (AP, that is) and the commensurate fodder - roughly enough to meet a third of the needs of McClellan's army.

This means that a force of 1/3 of McClellan's army at Orange (the "blocking force" or "rearguard" I've used in previous models) could be considered to be able to sustain itself indefinitely but that this takes up the great majority of the capacity of the line. This is one reason why McClellan could be reasonably expected to want to shift east to Fredericksburg to regain supply.
 

Dead Parrott

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Speaking of wagons, one important constraint is the amount of food that could be supplied to the army before the next move between supply sources.
My suspicion is that it could be reasonably modelled that the army had full wagons at Warrenton (because the army was able to sustain itself there for more than a week after McClellan's relief and then march to Fredericksburg north of the Rappahanock) but that once the army moves south of Warrenton Junction the section of line between Warrenton Junction via Culpeper to Orange should be modelled as having only the capacity for ca. 50,000 man-days of food per day (AP, that is) and the commensurate fodder - roughly enough to meet a third of the needs of McClellan's army.

This means that a force of 1/3 of McClellan's army at Orange (the "blocking force" or "rearguard" I've used in previous models) could be considered to be able to sustain itself indefinitely but that this takes up the great majority of the capacity of the line. This is one reason why McClellan could be reasonably expected to want to shift east to Fredericksburg to regain supply.
Folks consistently underestimate the capacity limitations - and therefore strategic impacts - of logistics.
 
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Saphroneth

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Folks consistently underestimate the capacity limitations - and therefore strategic impacts - of logistics.
Indeed - this is one reason why the Wilderness movement (across to Fredericksburg and Port Royal) of 1864 wasn't really feasible in early 1862, because there weren't enough wagons available to allow the overland movement (it's a bit iffy even in November 1862 when there's no real prospect of Lee delaying the Union army, simply because of the wagon count). It also colours the whole of the approaches to Richmond because by far the best supply routes are the tidewater rivers - viewed from this point of view it's almost inevitable that the Overland campaign hooked around and came at Richmond from the south.


In this particular case in November 1862 then the problem is "get over the Rappahanock and the Rapidan and get into a sustainable supply situation" - the only real option if the reports about the capacity of the O&A are correct is to get supply from the Fredericksburg region, though how to manoeuvre to get there is worthy of consideration.
This is also presumably why when Lee thought he couldn't defend at Fredericksburg he fell back to the North Anna position, because it controls the rail lines south to Richmond and gives Lee the interior line to fall back into the Richmond defences if a Union army hooks around to the east (via waterborne supply).
 

Saphroneth

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I should also note that Lee can't reasonably expect to make estimates based on the capacity of the O&A line. He has to treat it as if it has a higher capacity than the true value, because underestimating enemy capabilities is a much worse situation than overestimating them - if Lee makes dispositions which rely on the O&A only being able to support ~2 enemy corps instead of 3-4, and it can actually support 3-4, then he'd be in a bad way.
If on the other hand he can make dispostions which don't rely on the O&A's precise capability, he's fine.
 

Saphroneth

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I've decided to share what I've got to so far in terms of viable march routes for closing up with the Rapidan.



In previous posts I have largely modelled from the point of view that Jackson would be able to move fairly soon after the point of divergence; that is, that a few days after Lee abandoned Culpeper Jackson would be able to begin moving to unite with Lee.

This has been a position used for modelling on the basis that it is one acceptable to (indeed raised by) those who think that Jackson's pause was incidental and not reflective of his true ability to move; the fact that the modelling has indicated that Lee would be in a difficult strategic situation anyway is a bonus.

In this case, however, I will be representing a more middle of the road position in terms of Jackson's ability to move, because I will be modelling march distances and corps positions for both sides. I will assume that Jackson cannot or does not begin moving until three days after Longstreet abandons Culpeper, or the 16th of November - whichever is soonest. Note that the 16th is still two days before Jackson historically began to move, and it is two days after Lee's order to Jackson which stated that (1) there was no rear for Jackson to raid and (2) that Jackson should unite with Longstreet "the sooner the better".
Effectively in this model Lee's letter of the 18th November was the trigger for Jackson to finally begin moving, and Jackson begun moving three days later (on the 21st) and a comparable letter would be sent when Longstreet abandoned Culpeper.


I will be assuming in addition that the marching speed of the Union is nine miles per day on average and that all Confederate formations march at ten miles per day. I will be assuming that all Union forces have full wagons as of the 10th, that they can top off to full wagons from any point along the Warrenton branch and that the line south to Culpeper/Rapidan/Orange can provide supplies sufficient for about one third of McClellan's army per day. (Full wagons here represents the ability to operate as a flying column for at least nine days.)
For the sake of simplicity I will treat McClellan's army as consisting of six corps of equal strength (1st, 2nd, 5th, 6th, 9th, 11th). 1st, 2nd and 5th start at Warrenton, 9th at Waterloo on the Rappahanock, 6th at New Baltimore and 11th at Greenwich; this is a simplification but represents the "centre of gravity" of the corps. It's a matter of just a few miles as to where exactly they start and this will largely "come out in the wash".
Each corps is 23,550 AP or 15,700 Effectives.

I will assume that Longstreet abandons Culpeper on the day McClellan's forces bivouaced within a day's march of the city (that is, he marches out promptly the next morning). I will also assume that a corps forms a road column about 5-6 miles long and thus that two corps can fit down a road in a day (by staggering their marches) but cannot follow one another closer than ca. 6 miles, and that where I do not have a better map all marked roads on the OR map are viable march routes but that no roads not marked are viable march routes. (An OR map showing NE Virginia specificially is sufficient to prove that this is an underestimate of the amount of viable march routes.)
Each corps will be assumed to start the day closed up and end the day closed up, each component having moved nine miles (for the Union). Ditto for CS divisions or commands but ten miles per day.


The first topic will be the approaches to Culpeper.

Planned routes of march:
9th Corps - Waterloo-Amissville-Rixey's Ford- Rixeyville - El Dorado Mill - reach the Griffinsburg-Culpeper road at the crossroads.
1st Corps - Warrenton-Waterloo-Amissville-Rixey's Ford- Rixeyville-Culpeper
2nd Corps - Warrenton-Sulphur Springs - Jefferson - Brandy Station - Culpeper
5th Corps - Warrenton - Liberty - Rappahanock Station - Kellysville - Stevensburg
6th Corps - New Baltimore - Warrenton - Liberty - Rappahanock Station - Brandy Station - Stevensburg
11th Corps - Greenwich - Warrenton Junction - Elkton - Kellysville


10th November.

9th Corps: Waterloo to Amissville (5.2 miles) and then to Corvin's Cross Roads (to turn left). The nine mile point is roughly at the schoolhouse on the route towards Rixey's Ford.
1st Corps: Warrenton to Waterloo along the contemporary route (now a back road) is 6.8 miles. 1st Corps is following 9th Corps at a distance of just under 7 miles, and encamps between Waterloo and Amissville (40% of the way to Amissville from Waterloo).
2nd Corps: Warrenton via Sulphur Springs (now Farquier Springs) to Jeffersonton is 9.5 miles. 2nd Corps encamps on the outskirts of Jeffersonton.
5th Corps: On the modern route via Opal (which seems to be the historical Fayetteville) - a route which goes right past "Liberty Drive" at about the right spot for the historical Liberty - it's 10.8 miles from Warrenton to Bealton Station itself. Reducing the distance down to 9 miles seems to be pretty much Liberty itself (meaning the junction). 5th Corps encamps at Liberty.
6th Corps: New Baltimore to Warrenton is about 7 miles (ignoring the modern roads and following a more contorted route). 6th Corps makes it about two miles out of Warrenton towards Liberty, and camps seven miles behind 5th.
11th Corps: The modern route from Greenwich to Calverton (the modern location of Warrenton Junction) is about nine miles. Based on the contemporary map this looks like it's pretty close to the historical route, so for simplicity I'll simply treat it as a single nine mile march.
Marches of the 10th.jpg


11th November.

None of McClellan's forces are within ten miles of Culpeper yet; Longstreet stays put.

9th Corps: The march to Rixeyville is 4.6 miles and then 9th Corps continues via Eldorado Mill towards the Culpeper-Sperryville Pike. The total distance from the halt on the 10th to the junction that's the 9th Corps target is 13.3 miles, and 9th halts just south of Eldorado Mill.
1th Corps: Warrenton via Waterloo and Amissville to Rixeyville is 13.6 + 6.8 miles, and 1st Corps halts not far short of Rixey's Ford.
(n.b. the configuration here suggests 9th Corps may delay a bit or 1st Corps march harder so 9th Corps is not "dangled over the river" by itself. This could be made up for with harder marches or delays on the 12th.)
2nd Corps: 2nd Corps marches half a mile into Jeffersonton and turns south. Jeffersonton to Brandy Station via the modern routes is 11.7 miles (going through Alanthus) but this is a longer and less direct route than the one on the period map; the route on the period map appears nearly straight. The straight-line distance is 9.3 miles, so I will assume that the actual length of the route is such that 2nd Corps makes it over Welford's Ford and halts at the crossroads north of Brandy Station.

5th Corps: for simplicity 5th Corps will march to Bealton and then to Rappahanock, and from there towards Kellysville. It's 1.8 miles to Bealton (and a bit further to the station), and then 4.8 miles to Rappahanock Station; from there to Kelly's Ford is another 3.4 miles. 5th Corps encamps two miles short of Kelly's Ford.

6th Corps: follows behind 5th Corps and encamps at Bealton Station.

11th Corps: Warrenton Junction to Elkton is 9 miles, camps there.

Marches of the 11th.jpg



12th November.
Longstreet abandons Culpeper, making for Rapidan and thence Gordonsville.


With a ten mile march he makes it to the area of Mitchell's Station.

This means that march orders need to be re-evaluated. The goal of the Union army is now closing up to the Rapidan and crossing it successfully.

Concept of operations:
9th Corps swings wide to Madison Court House (to push Walker back off Roberton's River) and then makes for Madison Mills (to cross the Rapidan).
1st and 2nd march through Culpeper. 1st then marches for Madison Mills (to cross the Rapidan as part of Burnside's Grand Division) and 2nd makes for the area of Rapidan itself.
5th aims for the fords downriver of Rapidan (Sommerville's Ford and Racoon Ford).
6th and 11th move on Germanna Ford to march straight from there to Fredericksburg.

Thus Burnside's GD crosses the Rapidan upstream of Rapidan Station and Porter's GD does so downstream of Rapidan Station.

Planned routes of march:
9th Corps - El Dorado Mill - reach the Griffinsburg-Culpeper road at the crossroads - James City - Madison Court House - Madison Mills.
1st Corps - Rixey's Ford- Rixeyville - Culpeper - Robertson's Ford - Madison Mills.
2nd Corps - Brandy Station - Culpeper - Mitchell's Station - Rapidan Station (north of river)
5th Corps - Kellysville - Stevensburg - Racoon Ford
6th Corps - Rappahanock Station - Brandy Station - Stevensburg - Germanna Ford
11th Corps - Elkton - Kellysville - Germanna Ford.

One odd result of this is that 6th and 5th Corps arguably march further than if their roles were reversed, but 2nd and 5th are operating as a single wing and 6th and 11th are operating as a single wing.
The likely result of this set of manoeuvres is that Longstreet will be compelled to withdraw from the line of the Rapidan and pull back at least to Orange if not to Gordonsville. Burnside's Grand Division will cut in behind him if he tries to hold Rapidan Station, while if he puts his main effort into resisting the advance from Madison Mills then it will be possible to attack him from the north.
(More detail will be given on this when we switch to the Orange County map.)


12th November continued:

9th Corps advances halfway from their halt point on the 11th to Madison Court House. (It's 18.5 miles from one to the other.) This sees their halt a few miles from James City.
1st Corps marches through Rixeyville and into Culpeper. (They have right of way over 2nd Corps.) They make it through the town but not quite to the crossroads where they will turn right (west).
This confirms the decision to have Longstreet retreat today - otherwise he would be attacked in Culpeper.
2nd Corps marches to Brandy Station and then to Culpeper, where they wait for 1st Corps to pass through first. (It could equally be argued that well timed marches would mean that 2nd could be through before 1st arrived, but I will assume maximum friction here.) This sees them march less than the full nine miles.
5th Corps marches two miles to Kelly's Ford, and then another seven miles to Stevensburg. The modern route takes a jack-knife detour and thus takes more than seven miles to get from Kelly's Ford to Stevensburg, but it appears that following the original route indicates that 5th Corps can camp on the outskirts of Stevensburg (even if not quite at the crossroads).
6th Corps moves to Bealton Station and marches via Rappahanock Station to Brandy Station. This is a journey of about nine miles (the exact distance depends where in the Bealton Station area 6th Corps bivouaced).
11th Corps marches for Kelly's Ford and bivouacs one mile from the ford itself, on the route 5th Corps was earlier in the day.
Marches of the 12th.jpg


13th November:

Walker has to make a decision today about whether to resist 9th Corps - even if they didn't earlier, the US cavalry will be reaching Madison Court House today. My assumption is that he will march to Madison Mills instead of trying to resist an attack by a force more than four times his strength.
The shortest route is 11.1 miles, so Walker will get most of the way there on the 13th and cross the Rapidan on the 14th. (If he pushes he will cross the river on the 13th; either way, he has broken clean.)
Longstreet marches the rest of the way to Rapidan Station. He does not need to choose how to defend the line of the Rapidan just yet, but he will need to soon - i.e. during the 14th. I will examine the constraints on his defence on the 15th when we switch to the Orange County map.

9th Corps marches to the crossing of Robertson's River. They cross, but do not quite reach Madison Court House junction.
Sanity check: Waterloo to Madison via Amissville and Rixeyville is 36.2 miles to the junction.
1st Corps marches for Robertson's Ford, aiming for Madison Mills. Their total distance to the latter is 16.2 miles, so they will get there tomorrow afternoon (ahead of 9th Corps) and will thus cross first on the 15th.
2nd Corps marches for Mitchell's Station. They wait until 1st Corps has cleared the town before beginning their march and then take a parallel route along less major roads, and do not quite reach Mitchell's Station by the end of their march. (Their objective for the 14th is Rapidan, which they can make with another 6-7 mile march; thus they will reach it by the end of the 14th.)
5th Corps marches to Raccoon and Sommerville Fords, a distance of eight miles from Stevensburg. They could theoretically cross on the 14th, but will wait a day so the movement is coordinated (and so I can switch to the Orange County Map consistently...)
6th Corps marches from Brandy Station via Stevensburg towards Germanna Ford. They must interrupt or delay their march until 5th Corps has finished exiting Stevensburg, but the total distance from Brandy Station to Germanna Ford along the modern route is only 12.7 miles; even with how the roads have changed it cannot be more than 15 along the period routes. I will assume 6th passes through Stevensburg and halts six miles from Germanna.
11th Corps heads one mile to Kellysville and then turns for Germanna Ford. It's nine miles from Kelly's Ford to Germanna, so they stop one mile short of the Germanna crossing.
Marches of the 13th.jpg


14th November:

Per the rules I previously established, this is when Jackson starts moving.
There are two possible routes that Jackson could take. One of them is to make essentially his historical movement.
Jackson's command HQ went:



21st: troops started leaving Winchester
22nd: Jackson and his HQ marched and went to Old Stone House
23rd: Mount Jackson
24th: Hawksbill
25th: through Fisher's Gap to Madison Court House
26th: having gotten ahead of the troops, near Madison Court House
27th: Gordonsville, redirected to Fredericksburg
28th: Orange Ct Hse
29th: Rode ahead of troops to Lee's HQ on the Mine Run Rd
30th: recce'd the area his divisions were assigned to
1st: Met the lead division at Massaponax Church


The important data point here is that on the sixth day of movement Jackson waited for his troops at Madison Court House; thus we can assume that Jackson's troops reached Madison Court House on the sixth march.
If Jackson took this movement with his first march on the 14th, his leading edge would reach Madison Court House on the 19th (presumably this would be DH Hill's troops, who started at Front Royal and had the least distance to march).

If on the other hand Jackson followed his previous instructions from Lee and marched via Swift Run Gap to Gordonsville, he would be delayed by several more days at least and arrive in the Gordonsville area.
Interestingly even if Jackson moved the moment that McClellan's main body began to march out of Warrenton (which is functionally impossible, he cannot get that information that quickly) and took the shorter route his troops would still not reach Madison Court House until the 15th; no matter how soon Jackson moves, he cannot arrive in time to help Longstreet hold the line of the Rapidan.
We can also assume that McClellan would have this estimate; in other words, McClellan would be able to tell that Jackson could not interfere with his operations in time.

Movements of the 14th:

9th Corps: marches to two miles short of Madison Mills.
1st Corps: marches to Madison Mills, and probably has time for a recce. (No marked road past Robertson's Ford on the detail map, but this seems to be because it's a map of the wrong county. The outline map and a map of Madison County both show it.)
2nd Corps: marches to Rapidan Station, and has time for a recce.
5th Corps: has time for a recce at Sommerville's Ford and Raccoon Ford.
6th Corps: marches to the Germanna Ford area, waits behind 11th Corps.
11th Corps: marches to Germanna Ford and has time for a recce.

Marches of the 14th.jpg


We will now be ready to switch to the map of Orange County.



I will also provide a map of Orange County with the current positions of McClellan's corps "just off the map"; I invite people to suggest how Longstreet disposes his forces.
His total available strength is that his main body has 23,400 Effectives or 27,600 Present, Walker (retired from Madison Court House) has 4,200 Effectives or 4,900 Present, and as of the 10th half of Hood's division was at Gordonsville in smallpox quarantine (at 3,250 Effectives or 3,900 Present). This puts Longstreet as having 30,850 Effectives or 36,400 Present; I'm including both numbers because the ratio between these two is not the same as the Union ratio, and so we should consider both Effectives and Present for measuring to what extent Longstreet can fight.

As a reminder, each corps of Union troops is assumed to be standardized at 23,550 AP or 15,700 Effectives. This means Longstreet is best modelled as 50% to 80% stronger than one corps but weaker than two.

Start of the 15th.jpg



So: where does Longstreet place his troops? Any suggestions?

(If anyone thinks that Longstreet could oppose the corps movements I've outlined in this post, I'm also interested to hear it. The way I've had the Union corps moving is specifically intended to allow them to curl in behind any of Longstreet's possible defensive positions north of the Rapidan, but I may have missed something.)
 
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Saphroneth

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I'd suggest Longstreet would march to Hanover Station via Gordonsville. This is ca. 87 miles.
From Rapidan Station itself to Hanover Station via Gordonsville is 75 miles, so 7.5 marches. (The two routes from Culpeper to Hanover Station via Gordonsville are basically the same length by the modern map whether you cross the Rapidan at Madison Mills or Rapidan Station.)

So basically abandon defending the line of the Rapidan (possibly after holding Rapidan Station and Madison Mills until 5th Corps starts threatening to unzip) and head straight for the North Anna because of the threat of 6th and 11th Corps getting there first.

Let's do a quick check of that... 6th and 11th Corps are crossing the Rapidan unopposed at Germanna Ford, and from there via Wilderness Tavern and Fredericksburg to Hanover Station is 58 miles.

I actually think that means that Longstreet can't even try and delay McClellan at the Rapidan. From the moment the leading edge of Franklin's wing crosses the Rapidan to the point they reach Hanover Junction is 6.5 marches (15th 16th 17th 18th 19th 20th and they arrive halfway through the march on the 21st) so Longstreet has to reach Hanover Junction roughly by the end of the 20th or he's lost his chance and Franklin makes it to Richmond first.

Working backwards this means that if Longstreet starts from Rapidan Station towards Hanover Junction it has to be early on the 14th at the absolute latest (14th 15th 16th 17th 18th 19th 20th and he arrives halfway through the march on the 21st). This is cutting it incredibly fine because it means basically a meeting engagement, so Longstreet has to already be marching south during the 13th.

This means that Longstreet simply does not have the time to spare to defend Rapidan station even a single day - in this simulated campaign Longstreet doesn't reach Rapidan Station until the 13th, so he has to keep going south. (Functionally this means covering 87 miles in nine marches - 12th 13th 14th 15th 16th 17th 18th 19th 20th.)

It might even be necessary for him to rail some troops down the Virginia Central from Gordonsville to Hanover Junction to be sure that there are troops there before Franklin arrives, because I did have Franklin's wing delay a little so everybody crossed the Rapidan at the same time. If Franklin crossed the Rapidan as soon as he reached it then 11th Corps is eight miles past Germanna by the start of the 15th, and he covers the remaining fifty miles in 5.5 marches (15th 16th 17th 18th 19th and halfway through the 20th.)


Either way, though, this means that Hood (who was at Gordonsville when all this started) is probably marching straight for Hanover Junction and the rest of Longstreet's corps is doing the same. With no significant Confederate opposition McClellan can move his troops east to regain supply from Fredericksburg pretty much in peace, and if he reaches Hanover Junction before Jackson then Longstreet is getting crushed at about 4:1.

Worst case scenario for the Union following this operational sequence - no matter how soon Jackson moves - is probably that both Longstreet and Jackson reach Hanover Junction before McClellan's main body does. Best case, Longstreet doesn't notice Franklin's wing for a couple of days, Franklin reaches Hanover Junction first.
 
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Saphroneth

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I'm now going to consider the matter of supply.

6th and 11th Corps (Franklin's Wing) only take eight marches from the 10th (full wagons) to reach Fredericksburg. They're fine, McClellan's army spent nine days operating as a flying column during the historical movement south from the Potomac.

Assuming both Porter's Wing and Burnside's Wing congregate at Orange on the 15th (i.e. that the Rapidan is crossed on the morning of the 15th for most corps, but that 5th pushes over the river a day early) then they've been on the move for six days. Orange is their last chance for resupply before marching to Fredericksburg, but two of the corps already touched at Culpeper on the 12th-13th and 5th got a chance to fill their wagons at Bealton on the 11th; 2nd was resting just north of Rapidan Station on the 14th.

Supplies provided to Union corps past Warrenton Junction:
10th - none (and assume no stockpiling)
11th - two corps-days of supplies to 5th Corps at Rappahanock Station. 5th Corps refills wagons and their personal timer starts on the 12th.
12th - two corps-days of supplies to 1st Corps at Culpeper
13th - one corps-day of supplies to 1st Corps at Culpeper (in the morning) and one corps-day of supplies to 2nd Corps at Culpeper (in the afternoon).
14th - two corps-days of supplies to 2nd Corps just north of Rapidan Station

Supply states on the 15th, counting the consumption on the 15th but before resupply on the 15th.
9th Corps - six days below full. (At least three days supply left.)
1st Corps - three days below full. (at least six days left)
2nd Corps - three days below full. (ditto 1st)
5th Corps - four days below full. (At least five days supply left.)
(6th and 9th are six days below full but will reach Fredericksburg before long.)

Distance from Orange to Fredericksburg: ca. 40 miles, so four full marches and one partial march. There are two parallel routes to Fredericksburg - the Turnpike and the Plank Road - and the 40 miles figure uses the Plank Road figure.
9th Corps picks up two corps-days of supplies at Orange on the 15th and marches out behind 1st. The wings can share supply to some extent so they both have 11 corps-days of supply left at minimum to make the march.

With marches on the 16th, 17th, 18th, 19th and partial 20th, McClellan's whole army reaches Fredericksburg and refills their wagons there for the next movement.
 
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Saphroneth

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The next question is - where is Jackson at this point? (I'll count arrival times from the leading edge here, though Jackson's column was a few days long.)


Counting from the day Jackson starts moving, day six sees him reach Madison Court House. I've seen a suggestion of day seven Gordonsville (that could be just the more mobile HQ or the leading division), but day 11 is when the lead division arrives at Fredericksburg.
If Jackson's divisions went Front Royal-Madison-Orange-Fredericksburg the length of the march route is about 98 miles, including Gordonsville instead it's 111 miles. The distance from Front Royal-Madison-Gordonsville-North Anna is 117 miles so at a bare minimum this is an eleven day march for Jackson and could quite easily be more (if Jackson's leading edge division went via Orange and took the full 11 days to march 98 miles this is nine miles per day, in which case the march to the North Anna position is thirteen days).

This means that the following situations obtain for Jackson's movement:

Confederate best case
If Jackson sets off literally the same day McClellan does (first march on the 10th) and we use the upper bound estimate of his march speed, then McClellan's whole force is at Fredericksburg supplying (except for Franklin's wing which is already halfway to North Anna) at the time Jackson's leading edge reaches the North Anna position - the end of the 20th. This is unrealistic because Jackson would have already moved on the 10th historically if he was going to do that.

Confederate middle of the road
If Jackson sets off as soon as possible after news McClellan is advancing could reach him (McClellan's movement detected on the 10th, two days letter travel time so Jackson gets the letter on the 12th, Jackson starts moving on the 13th) and we use the middle of the road estimates for Jackson's marching speed (to whit, 12 days to reach the North Anna) then Jackson's troops start reaching the North Anna area on the 24th (and arrive 24th-26th inclusive)

Confederate reasonable worst case
Jackson sets off on the 14th and we use the lower bound estimate of his marching speed, so Jackson takes 13 days and begins to arrive at the end of the 26th.

Confederate disaster
Jackson sets off on the 15th and takes the route via Swift Run Gap, per Lee's repeated orders. This is 133 miles for his leading edge and Jackson takes 13-15 days to make this journey, meaning Jackson begins to arrive on November 27-29.

(If Jackson can't even move until the 21st then he has no physical way of reaching the North Anna before December 1 - even this "disaster" potential has him moving six days earlier than he did historically.)



The distance from Fredericksburg to Hanover Junction is about 33-36 miles. This means that the time when McClellan could reach the North Anna with a given formation is on the fourth day they march out of Fredericksburg, and that Franklin's Corps (which contacts Fredericksburg on the 17th and can march out again on the 18th) closes up to the North Anna by the end of the 21st. The rest of McClellan's army (contact Fredericksburg 20th, march out again 21st) closes up the North Anna by the end of the 24th.

Assuming he marches straight via Gordonsville, Longstreet reaches Hanover Junction on the 20th (at the cost of not being able to hold the Rapidan) and Franklin gets to the North Anna the next day. A battle would be iffy (either side could win), but we'll assume Franklin is under explicit orders not to fight if there's significant strength present; he can certainly do a recce though and McClellan would be arriving into a situation where he knows a lot about the enemy dispositions and the lay of the land.


So, what's the situation on the 25th?

Confederate best case: whole Confederate army present, whole Union army present.

Confederate middle of the road:
Whole Union army present. Some of Jackson's divisions have arrived, more will arrive during the day and the rest will arrive at some point tomorrow.
An attack on the 25th or even the 26th is basically "another Antietam" with McClellan's army concentrated and facing most of Lee's army but a division or so of Jackson's corps yet to arrive. Only problem is McClellan's army is significantly larger than it was at Antietam and Lee took over ten thousand casualties of various types at Antietam; instead of McClellan deploying 87,000 troops PFD he's deploying about 115,000 PFD (number calculated from ORs for Burnside's "army in the field" November 10, minus Slocum and Morell) for a 31% increase.
Of course, McClellan might opt not to make the head on attack; see below for the manoeuvre potential in this situation.


Confederate reasonable worst case:
Whole Union army present. Jackson is not going to arrive until the end of the 26th, so Longstreet has to endure two days of attacks by himself.
Longstreet gets steamrollered. He is outnumbered 3:1 or so and there are several possible crossing points to come at him from the north, east and west.

The Confederate disaster scenario is pretty much the same except that Longstreet has to hold out even longer.



The manoeuvre potential of approaching the North Anna


Fundamentally the manoeuvre potential offered by approaching the North Anna is that there are several possible ways to cross it. The big prize is White House Landing, attractively positioned at the head of Navigation up the Pamunkey, but we don't want to give everyone deja vu so let's look at the other possibility.
Hanover Court House has a ford and a bridge that can be reached by quite major roads (for the area). Taking it cuts the Virginia Central and it's not far by marching from cutting the RF&P as well, so there's a very real potential of either cutting in behind Longstreet and ruining his defensive lines at the North Anna, or forcing him to spread over a wide area to cover the frontage from Hanover Court House up the Pamunkey and the North Anna past Hanover Junction to the crossing points at Jericho Mills etc.

Union troops get over the river at Hanover Court House, they've got your supply line and line of retreat largely cut off (and the inside line to Richmond). They get over the river at Jericho Mills and Quarles Mill, they've got you cut off from whatever component of Jackson's command hasn't arrived yet. They get over the river in the Hanover Station area itself and they've possibly done both.


Union worst case scenario, Jackson arrives in time that Lee can hold the North Anna position, as in hold all the crossing points from HCH to HJ in sufficient strength (notably something Lee doesn't appear to have done historically). Now McClellan just has to decide whether to incline right and try and take up his June siege lines or set up new ones right here and blast his way through Lee's positions, he is after all on a rail line and has the longer ranged guns.
 
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