“Fort Sumter was South Carolina territory and after secession they had every right to evict a foreign army.”
So, it is your view that US military installations in foreign nations can be attacked with impunity?
Well they didn't just attack it (like Perl Harbor). They negotiated and tried for months to get all of the federal forts vacated. They were successful until Lincoln decided he wanted war and refused to give up this one.
If the current German government or British government were to tell us to vacate the US bases on their territory wouldn't we be obliged to go?