Two POD suggestions
1 (this one's for 67th)-Is there any way McClellan could have won the war for the Union? What would he have had to do?
Several PoD's possible here. The most obvious is Lincoln doesn't interfere with the Peninsula plan and an amphibious movement up the York cuts off the ANV and destroys it in April 1862.
A more interesting one is that things continue as per normal, but Lincoln doesn't feel able to relieve McClellan in November 1862. I'll have to do this in parts.....
(All events in this post as per OTL)
In the aftermath of the Maryland Campaign McClellan's army had to refit for a month. Desperate pleas to "do something" from Washington were met with replies of "then supply me" from the Army. Refitting the army took longer than it should have, but finally on 22nd October McClellan issued orders to cross the Potomac and crush the rebel army, using the plan of action agreed with the President. He would move east of the Blue Ridge, seizing the gaps and boxing Lee's army up in the Shenandoah Valley. The president approved and released substancial reinforcements to McClellan, mainly the 3rd and 11th Army Corps (they never arrived OTL). At this time McClellan started to reorganise the Army of the Potomac into 4 "Grand Divisions"*, each of approximately the size of a European Corps**. McClellan's forces would cut lose from his supply lines*** and cross at Harper's Ferry and Berlin, move south to the east of the Blue Ridge aiming for the Manassas Gap Railroad some 30 miles south of the Potomac, where they would resupply. He would then cut lose again and head south and east for Warrenton and Culpepper to resupply again from the O&A railroad.
On 25th October, after positioning his forces, McClellan's army started crossing the Potomac with only 5 Corps (another corps was left, on the Presidents orders, to cover Sharpsburg etc.). Porter's wing, consisting of his own Corps and Couch's, crossed at Harper's Ferry. Burnside's wing, consisting of his own, Reynold's and Franklin's Corps. Each wing was supported by one of the two available cavalry brigades**** By the night of the 27th Burnside was at Lovettsville with 9th Corps and Reynolds had reached Berlin. Couch's corps had completed their crossing at Harper's Ferry and was concentrated. On the 28th Reynolds and Porter started crossing the Potomac. The the cavalry brigades ranged forward to recce the area and seize key positions.
On the 29th Couch began crossing the Shenandoah. Reynolds' Corps crossed the Potomac whilst Burnside edged forwards. Movements were disrupted when scouts reported Lee was moving on Harper's Ferry. This delayed Porter and Franklin from crossing the Potomac until the 30th due to the possibility of needing to concentrate against Lee. By the end of the 31st 4 Corps were in the Bull Run valley, excepting a division Porter left to guard Harper's Ferry, and Franklin was at Berlin. Pleasonton's cavalry had advanced to Snicker's Gap and had encounter a large force of Confederate infantry (a division of Jackson's Wing) holding it.
On the 1st November McClellan's forces marched into striking position against Snicker's Gap. 2nd Corps took it without major opposition the next morning, but provoked a major concentration against them. The whole of Jackson's wing intended to attempt to regain the gap the next day, but the Federal position was so strong that even Jackson couldn't conscience it.
Meanwhile, the movement to seize the gaps further south continued. Ashby's Gap was seized on the 3rd in a major cavalry action. (The largest all cavalry battle until OTL Brandy Station).
However, Lee's response to McClellan was to split his army. Lee remained in the Shenandoah Valley with Jackson, but despatched Longstreet with his wing to Culpepper. The forward elements of his wing arrived on the 3rd, and by the 5th Longstreet had established a good defensive position.
OTL this is what caused Lincoln to relieve McClellan. When he learnt that Longstreet was at Culpepper he overreacted. This is the PoD here, but it will not effect events until the 7th November.
Meanwhile on the 5th the change of base to the Bull Run Gap Railroad was completed. McClellan's advanced elements had operated as a flying column for 10 days. Reynolds seized Warrenton on the 6th (a day early) and McClellan formulated a plan to advanced down the O&A railroad, strike and destroy Longstreet at Culpepper, then continue the advance by changing base to the Richmond, Fredericksburg and Alexandria Railroad. Orders were given on the night of the 7th and on the 9th the disposition of McClellan’s army was:
Warrenton: 1st, 2nd and 5th Corps, reserve arty and GHQ
Waterloo, on the Rappahanock River: 9th Corps
New Baltimore and Thoroughfare Gap: 6th and 11th Corps
Coming down the O&A railroad: 3rd Corps (the head had reached Warrenton Junction)
(Note: these are the real dispositions, McClellan’s orders were followed for the marches of the 8th and 9th).
Lee had totally misjudged the situation, and on the 7th November was only just planning for a possible junction between Longstreet and Jackson, believing that only a detached Corps or so confronted Longstreet (see his letter of the 7th: OR 1, 19(2) pg 703).
Longstreet was so lacking in situational awareness that in fact he decided to attack the advancing Federal force, believing himself to be facing a detachment rather than 7 Army Corps.
(end OTL)
* Although attributed to Burnside, the "Grand Division" organisation was McClellan's, and came into force on 30th October. He had done this before on the Peninsula when he found Keyes was simply not up to the job, and formed a wing of 2nd and 4th Corps, effectively putting Sumner over Keyes. The confusion in command of the Maryland campaign seems to have convinced him that a General could not hold both a Corps and Wing command, and so decided to formalise the arrangement. As an interim the army was divided into 2 wings, under Porter (standing in for Hooker, wounded) and Burnside.
** There are good reasons for this size. A column of ca. 30,000 men is the most that can fit down a road in a day.
*** This campaign sees the maturation of the "flying column" method of supplying armies in America. The troops carried 7 days rations and made their marches out of supply, aiming for another resupply point. The best examplar of this in the ACW is Sherman's 1864 campaign.
**** What was "the Cavalry Division" in the Antietam campaign became 1st Cavalry Brigade. The cavalry of Pope's old army was remounted at Washington and became 2nd Cavalry Brigade.