So you're saying that the US should be able to keep military installations in other countries without permission?
Leaving aside the entire question of whether South Carolina was another country or not, Sumter was the property of the federal government. Southern secession, even if legal, did not change that. Wouldn't it have made sense to negotiate the disposition of federal property in the southern states before the acts of secession?
And for the record we do keep military installations in other countries without their permission. It's called Guantanamo Bay, Cuba.
___________________________________________________
US out of Cuba!