For TTL, I've decided to have independant CSA.
To achieve this goal, McClellan must be elected in 1864; a situation that could favor McClellan would be a CSA being in a better situation than they were during late OTL 1864.
A better stand of the Confederates in West, more than in Virginia, is in my opinion the only mean to reach the objective.
After having made searches about the ACW, the Heartland offensive has seemed to me a missed occasion that would have permit CSA to last longer.
I've understood than the lack of cooperation between Smith and Bragg was one of the main causes of the confederate failure.
I guess if a more cooperative general had led the Army of East Tennessee, an important victory would have been scored.
Bragg wished to take Nashville but was forced to follow Smith into Kentucky if I'm not wrong. Assume that this is achieved ITTL, Paducah seems logically the next target as it would remove, for some months at least, the threat of Union over Mississippi.
I guess too that the task of the Confederates would be made easier by the behaviour of Buell, a western McClellan it seems.
So, to eliminate Smith, we could make him mortally wounded at Bull Run. I think to Heth to replace him.
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The battle of Shiloh doesn't seem to me as important as the Heartland offensive, but the delay that a confederate victory would cause in the capture of Corinth (4 to 6 weeks) has to be not neglected.
In this purpose, keep Beauregard far from the battlefield of Shiloh is essential. Without him, so with Bragg as second-in-command of Johnston, the battle would have occured a day earlier, and the compact confederate columns would have destroyed the lines of Grant, while Buell would arrive too late.
However, I'm less sure about the behaviour of Johnston (if he survives) or of Bragg during the Corinth siege and if the Heartland offensive would still occur. Also, I've not decided the fate of Grant and Sherman among other.
I will wait to better evaluate the consequence of a CS victory at Shiloh.
As means to keep Beauregard away, I've thought to send him to East Tennessee, then to have him removed before the Heartland offensive, or to send him at Charleston instead of Pemberton who would remain loyal to Union.
A Confederate victory at Shiloh prevents a US capture of Corinth and would more than delay a siege of Vicksburg.
A CS victory would not prevent Halleck to come back with 100,000 men at least, and I doubt that Bragg or Johnston could halt them.
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An other idea I've had is to prevent Pope from going to East as I said in an early post:
In Virginia, the Peninsular Campaign goes as in OTL, but the Northern Virginia Campaign is less disastrous than in OTL for the Union as Burnside is named to command the Army of Virginia.
In september, the Maryland Campaign occures as in OTL, and still ends into a Union victory. Thereafter, Hooker become the commander of the Army of the Potomac, and he uses the winter to reorganize.
In earlier spring 1863, Hooker launches a new offensive into Virginia, using the strategy chosen in OTL by Burnside with more success in the beginning as he manages to cross the Rappahannock River before the arrival of Lee.
But the battle of North Anna is a disaster for Hooker (the ATL equivalent of Chancellorsville).
Lee launches in late spring his offensive into Pennsylvania. Stonewall Jackson is still alive, and Lee is more successfull than in OTL. Harrisburg is briefly occupied by Stuart. Jackson manages to win the battle of Gettysburg in extremis in just a day. Thereafter, Lee encounter Meade at the Pipe Creek Line. Meade manages to hold off the confederate assaults and Lee retreated into Virginia.
The Gettysburg Campaign was the result partially of Lee's idea to relieve pressure from the Siege of Vicksburg.
The Pennsylvania campaign (OTL Gettysburg campaign) could be a response to the renewal of the Union offensives in West, following the confederate reconquest of Tennessee.
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Other point: the blockade. After a Conservative victory during the mexican civil war (Reform War), I plan to have CSA using mexican ports to bypass the Union blockade. Is it possible? What would be the relations between CSA and Mexico during the TTL ACW? How would it affect the war?