Would the Union consider an offensive into the CSA to be the best option in these circumstances:
- The British appear to be standing on the defensive in Canada therefore limited screening forces and demolitions should be sufficient on that front
- The Union blockade of the CSA has been broken but it will take time for the Confederacy to import arms & re-equip/re-deploy forces.
- The Union itself is now being blockaded so the current armament/supply situation is the best it's going to be without significant time to replace imported supplies.
Therefore "On to Richmond!"
Or is this too much of a gamble?
It's almost certainly far too much of a gamble. The absolute minimum for screening forces (what with the
potential for the British to advance down the Richelieu-Hudson axis) is about five divisions less than I have them divert here - this relies on putting about one division each at all the points of contact and one each at the places the British could land troops, giving up any chance of preventing the British buildup, and fundamentally accepting that if the British advance they're quite likely to get to
New York - and that means the possibility for McClellan to concentrate twelve divisions around Washington, if he leaves one at Baltimore (to defend against the British, natch) and pretty much accepts that the CSA's got the initiative in the west.
Twelve divisions is, you'll note, less than what JE Johnston has.
Making things worse is that you simply can't use the same single field army to both protect Washington and attack into the CSA, and that riverine supply is functionally impossible. So now you need to leave some of your forces in Washington (at the time the estimates for how
much went as high as five divisions, and when McClellan went to the Peninsula they held back an oversize corps which I would count by my numbering as four divisions in addition to this) and thus the offensive manpower of the army could be limited to as low as three divisions (obviously unworkable) or as high as seven (still not really workable). But let's assume that for this campaign there's as many as nine divisions (100,000 men by the measure I've been using).
Now, the problem is that McClellan's forced to conduct an "overland" type campaign. He can't use even the Rappahannock for supply, so his supply base is going to be overland all the way from the northern water terminus of the Fredericksburg and Richmond railway - and there's good lateral communications for an enemy to strike against his sixty-five-mile-long supply route, even if he makes it to the gates of Richmond without fighting a battle.
But let's look at how it might go if there was a battle or two. Since his communications are completely impossible without that rail line, McClellan has to fight all the defensive positions along the line and he has to do so against superior forces. He can't turn the positions like Grant did because Grant used waterborne supply, so he has to fight head-on. Specifically, he has to fight at, say... Fredericksburg/Spotsylvania, the North Anna and Cold Harbor.
McClellan has no source of extra reinforcements and is outnumbered to begin with. If we assume that McClellan's substantially better in making assaults than Grant or Burnside (a factor of two) then we can calculate based on known casualty rates thus - assuming, of course, that McClellan additionally
wins the battles (that is, gains control of the contested ground and can continue his advance) instead of doing what Grant did and losing.
Spotsylvania/Fredericksburg.
The Spotsylvania battle OTL saw Lee's army, outnumbered 2:1, inflict 18,000 casualties while taking 13,000 of his own. This means each of Grant's ten division slices inflicted 1300 casualties and each of Lee's five division slices inflicted 3600.
In this battle McClellan's ten division slices inflict 2300 casualties each, for 13,000, and Johnston's thirteen division slices inflict 1800 casualties each, for 23,400.
Johnston's remaining army is 117,000 and McClellan's is 76,600.
(Using Fredericksburg, meanwhile, is even worse though more appropriate. Lee's army there inflicted one casualty for every six present and Burnside's inflicted one casualty for every 27 present. Doubling the inflicted Union casualties per man rather than halving the inflicted Confederate casualties per man, you get McClellan's army inflicting only 7,400 casualties and taking 21,000).
At the North Anna, things are less bloody - Lee's army inflicted one casualty per 12 men and Grant's inflicted one casualty for every 53. This barely moves the needle for Johnston's army as McClellan inflicts about 1500 casualties, but the Union general takes roughly 5,000 even with the doubled efficiency.
Johnston's remaining army is 115,500 and McClellan's is 71,000.
Then we have Cold Harbor. At this battle Grant's army inflicted one casualty for every 18, and took one casualty for every four in Lee's army. This means McClellan's army would take about 14,500 casualties and would inflict 4,000.
When McClellan reaches Richmond, he has an army that's been reduced down to roughly 56,500, and Johnston still has 110,000. And remember, I've been assuming he
won three battles which were tactical defeats for the Union...
The exact numbers will vary, but the point is pretty simple - McClellan's advance is forced by the constraints of the overland route to be parallel to the Confederate line of operations (that is, they can keep falling back to new fortified positions along their own supply lines) and he has no way to turn the positions because riverine supply is right out.
