A British fort in Boston Harbor WAS legitimate English property in 1777. Why would it not be?
The Brits didn't recognize American independence in 1777. They, and the colonists, made no bones about the fact that the colonists were engaged in a revolution -- just as the secessionists in 1860-61 made a revolution.
It needs to reinforced, I suppose, that Fort Sumter was federal property, not South Carolina property. The feds wouldn't agree to complete the fort ---unless--- clear title was conveyed, which it was in 1841.
The secessionists were no better than common thugs with a gun. In fact, they were common thugs with guns.
Walt
Cause of a little document signed in 1776. But then again, I guess things like that don't matter to persons such as yourself who take a "might makes right" view of the world in all cases except when might is exercised to expunge a hostile army from a strategic position inside the borders of the rebelling country.