Maryland secedes

[Anaxagoras]

I'm not convinced that Washington needs to be evacuated at all. Washington, as has been pointed out in a lengthy and well-researched essay on this very site, was nigh impregnable to the Army of Northern Virginia. Why would the fact that there are troublemakers in Baltimore (which can surely be put down) require a withdrawal from an impregnable fortress?

It wasn't an impregnable fortress until 1862.

 

[mowque]

Another likely development is that the First Battle of Bull's Run does not take place as the Union is occupying Maryland, instead deploying sufficient forces to hold Washington DC and the areas just over the Potomac in Virginia.

Instead of an early Confederate victory in battle you have one of the (now) twelve Confederate states crushed at the very start. Or, given the value Maryland would have brought the CSA perhaps it is the CSA which is forced to attack in hopes of saving Maryland and suffers an early defeat?

This. You'll start the CSA off with a battle it can't win. Not good for the newly formed state. And you are going to make the Union even more angry. So much for the 'self-defense' argument.