There was originally a plan for a coordinated attack in the morning of the 19th. But when Rosecrans fell behind schedule, the timetable was thrown up in the air.
Rosecrans sent messages that he would be in place by 2:00 in the afternoon. Grant looked at a map and thought that ETA was impossible, and so he told Ord to hold up until he heard a battle. The reason Grant thought that it was impossible is that he expected Rosecrans would be coming in on both roads.
Rosecrans was able to arrive and initiate the battle about 4:00 because he came in on the shortest route on one road, while leaving the other road open. He did not notify Grant that he was doing this.
So, in striving to have the battle before dark, Rosecrans decided to leave an escape route open.
I realize other posters have different theories on why Rosecrans left the Fulton road open for the confederates to escape, but my opinion it that he left it open because he was running so late and he was trying to make up time by coming in the shorter route. He thought wrongly that he could detach a force to the Fulton road once he got close to Iuka.
This is my version of the inexplicable.
I think it was indeed too risky to attack with two columns. Rosecrans panicked big time before noon when Sanborn had a squirmish with rebel cavalry. Before deciding to abandon Cartersville he had ordered it held at all hazards with instructions to find a strong defensive position. At the time he still thought to proceed with divided forces but he was crapping himself.
Castel is of the theory that the most important element Rosecrans needed to be successful was surprise. In fact he thought without surprise Rosecrans' plan would fail to trap Price. Ord's role was to attack first in order to facilitate that surprise. Getting as close as possible by night was also to enable the surprise. When Rosy was discovered before noon, that put the biggest wedge on his plans than anything else. He's not going to admit that however. He must blame someone else if all goes to **** and we'll cue the scapegoats.
Why all the ballyhoo and brouhaha about everything else? Denying the Dickey and Lagow change in orders is classic "The Mistakes of Grant" distortion of reality that Rosecrans is capable of. In order to defend himself for his mistakes he would distort the truth, reframe things, outright lie, and pin the malfeasance on Grant. What point is there for Grant to send Rosy not one, but two staff officers to give him pointless advice to hurry up? Rosecrans already knew he was running late and didn't need to be reminded. That's not the reason D&L we're there. They were there to make sure orders were obeyed. Also, what point was there to send D&L to Ord at 10:00 am with a change in orders and not send that same change to Rosecrans immediately after? The whole thing strains credibility and doesn't make sense. Then there's the Dickey letter to his wife.
Rosecrans lied about all that because he had to pin that failure on someone else, and when he testified in Congress he hated Grant. Castel is disregarding that entire absurdity that Grant didn't send Rosecrans a change in orders and so am I. Could that have been the reason Rosecrans decided to consolidate his columns like
@tony_gunter theorized and not the fact he was found out so early in the day? Sure, because Rosecrans was already crapping himself, D&L telling him that he now had to open the festivities would just add on to the doubts he was already having. He would comply with orders because he had to, but not in the way Grant thought with columns divided.
What about the map inaccuracies and Col. Milzner? This is the classic case of when it rains, it pours. As if Rosecrans wasn't already in a jam, his route calculations were wrong. Maybe they wouldn't have mattered at all had he been on time and moved in by night with Ord drawing the tough assignment of being bait, allowing him to spring the trap, but if he had to attack Price straight up? Specially when the latter was expecting it? Then the distance was insurmountable, too risky to separate the columns, etc
These things happened almost concurrently, and sequentially they were like a cascade of failures. Cozzens is of the opinion that Rosecrans had already decided to consolidate his columns before D&L even got into the picture. That could very well be because he eas already in panic mode after being discovered, and the roads and distances were longer and farther apart than thought. It's pretty much irrelevant in terms of final effect as all these things just add on to each other, o complicate an already panicky situation.
I am writing this in disorganized fashion but it could very carefully be reconstructed to show the record sequence of events. One thing just adds on to another like a cascade failure.
Why didn't Rosecrans notify Grant?
This is really the speculation, but I'll give you my opinion. That was common sense old school variety, CYA. By not leaving a paper trail, in this case he could still salvage his reputation, his narrative, and be able to fashion afterwards a positive report for himself depending on how things developed. This is a tactic of the old army game. If he left a written trail, it would be a written admission of his mistakes, they would show up on reports, and get magnified even. Everything that went wrong thereafter would get blamed on his delay and his leaving one road uncovered (rightfully, but notice how by not leaving a written account of that he was indeed able to spin things around to suit his purpose,) Besides, he was still hoping against hope that he could salvage the situation, by using the road connecting Jacinto to Fulton to still capture Price. He'd write to his wife within two days that if he got cooperation from Ord he would have been able to capture Price. He might have been mistaken but that's what he thought, so he would indeed have thought against reason the plan was still salvageable. Then why leave such an admission of errors at 12:40 pm that his maps were wrong that he couldn't cover both roads, etc?
I don't have anything to comment about delays, the roads, etc. that part I have to catch up on.
I'll revise this if shown incorrect as this is my working impression.
Bringing the