Maryland and the Emancipation Proclamation in 1861
The US loses sympathy in Europe for losing their capital city, and gains sympathy in Europe for abolishing slavery.
The only reason the US didn't abolish slavery in 1861 when Lincoln took office and the Southern Senators and Representatives left is because he was trying to save the Union by appeasing the slaveowners.
No Maryland, and slavery is gone immediately.
This leaves a Union army in the Cumberland and another on the shore, a strategic occupation first of the Delmarva peninsula, followed by blockade of the Confederate capital of DC and Baltimore.
This has another factor in logistics. The Army of Northern Virginia could rapidly move back and forth to guard Richmond from both the north and the east, from land invasion across the Potomac and sea invasion up the James.
Now they can't.
Maryland costs them an extra army. On the other hand, if they can extend conscription to Maryland they can raise another army. This kind of cancels itself out.