Uh, isn't Ft. Sumter in South Carolina? What were Northern troops doing in South Carolina when they weren't welcome and were given the chance to leave? Isn't a landlord allowed to evict a tenant who doesn't have a lease?
If Soviet troops had begun landing in New York and moving into apartments there, would New Yorkers have been "initiating a war" if they fired shots at the intruders? It would be ridiculous to say "the Soviets fought because the New Yorkers shot at them and initiated the war," wouldn't it?
Recall that New York and Virginia ratified the Constitution conditionally; i.e., only under the guarantee of the right to secede.
Is a landlord allowed to evict a tenant from a house he doesn't own? The island that Sumter stands on was not leased from South Carolina. It was owned by the United States government.
I'd also ask you what you would think if, say, the Cubans opened fire on Guantanamo Bay.
But the federal troops were there before the rebellion began so it wasn't an invasion. Sumter belonged to the federal government, not South Carolina. Absent an act of Congress, Sumter remained federal property.