Civil War arguments

[BCPVP]

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If the south had the right to leave the union, the Union forces clearly should have evacuated the fort, and their refusal was an act of aggression.

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Why? The forts were federal property. The South said "Get out, that property's ours now". The North said "Like hell it is." The South then opened fire on it. How did federal property magically turn into state property simply because the South seceeded? Seems to me the South initiated the aggression and escalated it by attacking the fort.