Fort Sumter was a Federal Fort when it was fired upon. This still constitutes as an attack on the Federal Government. You don’t “evict” the Federal Government with the firing of weapons; that was an act of war.
It was a police action by a sovereign state against a foreign army. And it in no way threatened the Federal Government. Washington could have continued indefinitely to exist and serve (or rule over) its remaining member states with or without a military outpost in the CSA.
Absolute hogwash. SC had seceded, and Northern troops and equipment were being removed from all over the South. Those Union troops who stayed behind at Ft Sumter were on foreign soil. There was no agreement about diplomatic missions, embassies, or anything else between the Union and the Confederacy, so the land and the buildings were clearly NOT Union possessions of any kind. The troops there were squatters at best, who refused to leave after repeated peaceful requests and efforts to remove them. At some point, law enforcement will use force to remove those who refuse to comply.