This is a question I've wondered about and never quite resolved in my mind, so I thought it'd be interesting to throw open.
Hypothesizing for the moment that through some means probably but not certainly involving foreign involvement (either in intervention or a concurrent war) the United States is not in a permission to stop the Confederacy becoming independent... what exactly happens to Washington?
I see roughly these options.
1) The new border is on the Rappahanock or the Occoquan.
This at least means Washington isn't a front line city, but it might not be something the Union can actually enforce.
2) The new border is on the Potomac, and the Delmarva is under Union control.
This makes Washington essentially a front line city, and it'd be far too vulnerable in any future war to really contemplate having as the capital.
3) The new border is on the Potomac, and the Delmarva or lower Delmarva is under Confederate control.
This means Washington is also pretty thoroughly cut off by sea.
4) The new border is around the Mason-Dixon line (i.e. Confederate Maryland and Baltimore), but Washington remains under Union control.
This makes Washington completely untenable as the Union capital. It could not possibly be fortified well enough to avoid capture on the first day of any future war.
5) The new border is around the Mason-Dixon line and Washington is ceded to the Confederates.
For this reason, as far as I can tell there are only two options - either the Union has enough pull in the peace negotiations to retain a border which means the Confederacy doesn't actually get all of even "non-West" Virginia, or there's a different Union capital when the dust settles.
So what city would end up the capital?
Bonus points if you can come up with a capital which the Union would be willing to go for if what actually swung the war was British involvement, because in that case I suspect the Union would quite like a capital that can't be bombarded from the sea...
By the beginning of 1862, the CSA Government was stating that it would not accept anything less than Maryland, Virginia, Kentucky, and Missouri. So if CSA won, DC would go to the CSA, so option #5
USA would probably move capital to Philadelphia
Not sure about Philadelphia. If Maryland goes with the Confederacy so probably does Delaware. This closes the Delaware River to the Union and Philadelphia becomes a land locked city near a possible hostile neighbor.
New York City would be a much more secure capital and have access to the ocean as well.
Along those lines I like to imagine the Republic of Tri-Insula not choosing New York City as its capital!For grins and giggles I go in the other direction: a truly major defeat for the Union is just that. Before long the (old) Northwest secedes from the Northeast; North Carolina, Georgia, and Tennessee secede from the new Confederacy, New York City declares itself independent, as does California and Texas...
For grins and giggles I go in the other direction: a truly major defeat for the Union is just that. Before long the (old) Northwest secedes from the Northeast; North Carolina, Georgia, and Tennessee secede from the new Confederacy, New York City declares itself independent, as does California and Texas...
LOTS of capital cities...