Something to keep in mind with the Ft Sumter thing. It was built following the War of 1812 by the Army Corps of Engineers. It wasn't so much a fort in SC that the Union army occupied, so much as both sides had some level of claim to it. The Union army had claim as its original builder and operator, plus the secession was considered to be illegal to begin with so they never relinquished claim over it. South Carolina's claim was that after the secession it was in sovereign territory and hence not under command of the Union army any longer.
The Union forces in Ft Sumter didn't come out of nowhere - they weren't marched down out of the north to occupy it or such. Sumter was one of 4 forts near Charleston. The forces that moved into Sumter 5 days after the secession actually came from Ft Moultrie, another of the Charlston forts - Moultrie was an older design (Rev. War era, as opposed to the post-1812 Ft Sumter) that was seen as indefensible if the Confederate army were to attack, so they repositioned. The 4 forts were all originally manned by the army, but the soldiers from 3 of the 4 surrendered immediately after secession rather than fight.
The Union forces in Ft Sumter didn't come out of nowhere - they weren't marched down out of the north to occupy it or such. Sumter was one of 4 forts near Charleston. The forces that moved into Sumter 5 days after the secession actually came from Ft Moultrie, another of the Charlston forts - Moultrie was an older design (Rev. War era, as opposed to the post-1812 Ft Sumter) that was seen as indefensible if the Confederate army were to attack, so they repositioned. The 4 forts were all originally manned by the army, but the soldiers from 3 of the 4 surrendered immediately after secession rather than fight.