Pennsylvania Campaign in 1862?

So in 1862, Lee crossed over into Maryland east of South mountain, and criss-crossed this way and that through the mountains and over the Potomac to capture the garrison of Harpers Ferry, culminating in the crisis of September 14 when McLaws's command was bottled up in Pleasant Valley. His initial goal had been to force a battle with the main Union field army by threatening Washington, Baltimore, or Harrisburg from Frederick Maryland, but he probably overreached for the opportunity to bag Harpers Ferry, and lost thousands of men to straggling as he scrambled to defend against McClellan's thrust through South Mountain.

So what if instead, he crossed the Potomac west of South Mountain and marched up the Cumberland Valley with his army of 69,000 veterans, with the goal of fighting a battle somewhere in southern Pennsylvania or northern Maryland east of South Mountain? Longstreet with Jones, Hood, and Walker could cross South Mountain through Monterey Pass, while Jackson could use Cashtown gap, with McLaws and Anderson marching on Harrisburg? He could bag the garrison at Martinsburg, which would probably dissuade the garrison at Harpers Ferry from any threatening action, and without major rivers or mountain ranges dividing his army, he could face McClellan's army without needing to desperately scramble his men into position, and thus deploy more men. At the time, about 18,000 men in the Army of the Potomac were brand new; they'd just completed their organization and equipment, and had conducted no training. Most of the rest of the army was still smarting from defeat at Bull Run, which was partially alleviated by the victories at South Mountain.
 

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It will depend how spread out the Confederate forces are with the advance on Harrisburg. It sounds like an 1862 Gettysburg. Are yo allowing for a revised Lost Order?
 
So OTL, McClellan left Washington on Sep 7, the same day Lee's main body entered Frederick. This was three days after the first elements of the ANV entered Maryland. McClellan then marched on Frederick, and arrived on Sep 13; this is about 6.5 miles a day.

It took Lee 10 days to get II Corps from Fredericksburg to Winchester; this is a distance of 94 miles, so about 9.5 miles a day. Starting at Centreville, with his left wing entering the valley through Manassas Gap and passing through Winchester, the distance is reduced to about 65 miles, so maybe seven days march instead of ten, then three to hit the garrison at Martinsburg. Left wing with two divisions under Jackson, Center of four divisions under Longstreet, right wing of two divisions under A.P. Hill.

The course of the campaign depends on what the tripwire is for McClellan to move. Does he leave Washington the day Lee arrives in force north of the Potomac, or three days after his army begins to move? It took Hooker until the 13th to respond to Lee's shift into the Valley and the attack on Winchester, so I think if Stuart does his job, McClellan may not sally out until September 14, when Lee strikes the garrison at Martinsburg after about ten days march at 1863 pace. By then, his right and center should be crossing the Potomac at Shepherdstown, and his left should be over the following day.

From Sharpsburg, Lee could put his right wing on the road through Boonsborough to Waynesborough then Fairfield (50 miles), his center up to Hagerstown then Waynesborough then Cashtown (55 miles), while his left moves from Williamsport through Chambersburg up to Harrisburg (80 miles).

Occupying Williamsport on the 15th, Jackson has 80 miles to march to get to Harrisburg with two divisions; using Ewell's pace between Chambersburg and Carlisle with two divisions as a baseline (15 miles/day), Jackson should be able to secure the surrender of Harrisburg by the 21st. By that time, Longstreet and Hill should be able to debouch from South Mountain and be in a position to concentrate for a stand around Gettysburg. Longstreet can get from Sharpsburg to Gettysburg in four days (Sep 18th; 20th if he marches 10 miles a day) at that pace, ditto Hill.

Reacting on the 14th, McClellan could try to force his way through the gaps in South Mountain, getting to Frederick by the 21st at his OTL pace, but by then Jackson is already menacing Harrisburg, while Longstreet and Hill have six divisions east of South Mountain. This relative positioning would leave Baltimore open and seriously endanger the capital, though, as well as his line of supply. If he makes the march, but is surprised by the pace of the rebel march through the Valley, he could march north along the Emmitsburg road (as a general axis) and meet Lee along a line between Emmitsburg and Taneytown around the 24th.

If he anticipates the speed of the rebel advance, I could see him making a lateral shift to Baltimore (20th), then use the North Central Railroad as a line of operations, with corps spread out between Westminster and Hereford, though the lateral roads on the Baltimore-York axis don't seem ideal. In this scenario, I could see Lee advancing on and concentrating at Westminster, and setting up a defensive position along Parr's Ridge if it's usable to await McClellan's attack along that branch of the North Central Railroad. From there, if McClellan's attack isn't overwhelming and is repulsed, Lee can send his left wing to swing around to Reistertown to cut McClellan off if he can wrest control of the the road that goes to Hampstead.

Alternatively, McClellan could try to spread his army out all the way from Relay House north of Baltimore and Freeland just shy of the Pennsylvania border, to advance on Westminster along convergent routes, but the lack of good lateral roads in the region could expose his columns to defeat in detail if Lee can take the initiative and beat one column before the others converge on Westminster.

If McClellan's attack at Westminster is repulsed, but his army is not destroyed, Lee can shift to Frederick, which can threaten both Baltimore and Washington, to force another battle with his already defeated enemy; after the defeat at Bull Run and the repulse at Westminster, a well handled operational battle by Lee might shatter the Army of the Potomac.
 
It will depend how spread out the Confederate forces are with the advance on Harrisburg. It sounds like an 1862 Gettysburg. Are yo allowing for a revised Lost Order?
Right; the Confederate advance on Harrisburg will be made with two divisions; enough to overwhelm any PA militia defending the heights south of the river. The Lost Order fiasco only occurred because the Army of the Potomac kind of retraced the Army of Northern Virginia's steps; TTL, they're advancing towards each other (kind of circuitously on Lee's part, granted), so the only way that could happen would be if the Harpers Ferry garrison crossed the Potomac at Shepherdstown or Williamsport to join McClellan's army, or if there was a really out there Union cavalry raid. Even OTL, it was a pretty astounding stroke of luck, though it's actual impact on the campaign is debatable; he was already poised to punch through the gaps in South Mountain from his position at Frederick, though being able to understand all the reports he was getting from all over Maryland/Virginia through the lens of Special Order 191 probably helped.
 
I suppose if McClellan's army enters a region where the lateral/convergent roads aren't the best, but the ground is relatively level, Lee could take a page from Napoleon, who marched 120 miles to Dresden in four days with 120,000 men along one road; if the Army of the Potomac is split into different columns, this could be a very powerful play.
 
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