But none of that is the crucial factor in the success or failure of the attack.
At Gettysburg:
Longstreet's attack was probably doomed before it was launched -- Longstreet certainly thought so.
For Longstreet's attack to succeed, the Confederate artillery would have needed to do much more in their bombardment. They failed for a bunch of reasons; the Yankees artillery was stronger and better supplied. This leaves the Confederate assault very vulnerable as it makes an exposed approach. I have walked that path with a descendant of a man who lost a leg there -- no one was shooting at us, but it is a long way for troops under fire.
Pickett's Charge arrives at the Union line already a bit battered. The Yankees are waiting for it. There is no surprise, no diversion, no misdirection. The Yankees have known for hours where the attack will come -- it is obvious from the artillery bombardment if nothing else. From the high ground, they can observe the advance. They can position reserves to meet the attack, support in case the front line is actually breeched. They actually advance a brigade and bring Kemper's Brigade under enfilade fire as they hit.
At Chickamauga:
Longstreet's attack is generally well organized and run -- but the #1 reason for the huge success is the Rosecrans-Wood fiasco. Without Rosecrans' error, Wood's would still have his division in line; without Wood's deliberate decision to obey an order he knew made no sense, Longstreet would have run straight into a division of infantry set to defend instead of an undefended gap in the line. (While we are at it, this would also mean Wood's division would be defending the southern portion of Rosecrans' line instead of moving north, away from the fighting, to join Thomas at the northern end.) Longstreet's attack would have hit hard, but the key factor in its' success was the gap in the Union line.
At Chattanooga:
The critical factor is not the length of the line on the ridge. It is the failure of Bragg's army to properly fortify their position in a two month period. Thomas' troops advanced bravely, but the Confederate defense is incompetently organized. In a war where it would soon be assumed an army could make itself virtually invulnerable to a frontal assault in three days by entrenching, just how does Bragg's army mess this up? (The steepness of the ridge is also a factor -- but neither Grant nor anyone else thought that attack would work before it went in and up the ridge.)
Once Lookout Mountain has fallen, Missionary Ridge position is certain to be outflanked and Bragg is too outnumbered to hold here. Sherman blows his attack on the northern end and Cleburne smacks him back with a division and a half -- and Grant has enough troops that Hooker can be thrown in as a diversion that dooms Bragg's Missionary Ridge position all by itself. Once Hooker gets astride the ridge and starts moving north, there is no defensive position that can be held.
Thomas troops are another diversion, designed to take pressure off poor Sherman in the North-- they take the center on their own initiative and pent up frustration. Sherman -- Grant's intended main blow -- is stopped completely by Cleburne (who also covers the retreat of the shattered AoT), the only reason the Bragg's army survives this disaster.
Also noteworthy in all this is the virtual non-impact of Confederate artillery on the battle. Throughout the war, the AoT artillery never seems to have a major impact on any battle. Considering that Braxton Bragg was a well-known artillery officer before the Civil War, I have always wondered how that could be possible, and Chattanooga is a position where they could have done great execution on advancing Union troops.
Longstreet misses this day of battle, having been sent to take Knoxville on November 4 in another fiasco. I can see the advantage of re-taking Knoxville -- but how is it a good idea to send troops away from Chattanooga while Grant is building up to break the siege of Chattanooga? How is it a good idea to swap Longstreet for the troops already headed for Knoxville, delaying that operation? If they just keep Longstreet at Chattanooga, Bragg has about 4,000 more troops to hold his position and Longstreet could be more competent than most.
Overall, Gettysburg is the only one where the attack has nothing special about it, no advantage to make it succeed. The only thing that could have done that was the artillery bombardment, the greatest ever seen in America to that time -- and it failed. After that, it is simply might-against-might, a head-on attack that surprised no one, advancing across open fields. No wonder it failed.
Chickamauga benefits from luck and terrain. Longstreet's attack approaches through covered terrain (no gauntlet of artillery fire to run). It hits a gap in the Union line (Wood knew an attack was coming on his front, but moved out of the way, leaving that gap -- he should have been court-martialed). If you can picture the troops in front of Pickett getting up and marching away minutes before the attack hits, then you can see these two as similar.
For anyone interested, there is a particularly good discussion of the problems with the Confederate defense of Missionary Ridge in McDonough's
Chattanooga -- a Death grip on the Confederacy, starting on page 182, which you can find here:
https://www.google.com/books/editio...ga+Death+Grip+Confederacy&printsec=frontcover
Chattanooga is a mish-mash battle. Bragg should have listened to Hardee and Cleburne, who wanted to retreat; instead he listened to Breckinridge and stayed another day. The failure of the AoT to prepare a better defense from September to November is the prime reason Thomas' troops can advance up the slope of Missionary Ridge so easily, but that terrain is also difficult for the defense (too steep).
If there is a factor that explains the difference between these three, it probably comes down to terrain, surprise, and luck.
- Pickett's Charge suffers on all three: an exposed advance across open terrain into heavy fire against a prepared enemy with no sign of luck.
- Chickamuaga benefits from all three: an advance through covered terrain against an enemy who is disorganized and unprepared because of luck.
- Chattanooga: an advance up the ridge through terrain that is covered against enemy fire because the Confederates have done a poor job of organizing it (unknown to the Union, who certainly did not expect it). That is a lot of luck, which generated surprise.