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.
There were negotiations going on about the disposition of the US Ft.- Lincoln was overriding those by attempting to resupply the Fort.
free dixie,sw