“I would attack someone who wouldn’t leave my property too”
Something that always irks me about “first shot” arguments is that they are taken to mean whoever is responsible deserves what they get. But not all casus belli are created equal. You are allowed to fight for recompense for stolen or damaged goods, and also to punish wrongdoing. But of course conquering and forcing submission under the Constitution forevermore is not just punishment for Ft. Sumter.
It wasn’t meant to be, anyway. The Civil War was fought on the pretext that the Confederacy was a conspiracy of treason against the just laws of the federal government, and that all participants and sympathizers were insurrectionists. Ft. Sumter was merely a symbol for the pretended that the North was in some kind of danger. Which is a lie. The blessed union was in danger, but not whatever would have been left of the union had the South gone in peace.
Considering that the pretended confederacy declared war on the United States, it was certainly a war begun by the South. The South called for 100,000 soldiers before Lincoln called for 75,000. There was no reason for the south to withdraw. It was a temper tantrum. The slave power demanded the right to throw a temper tantrum when ever they wanted.
It doesn’t have legal merit. It doesn’t make sense. That is, still what they did.
By contrast, in 1775 England pretended the right to tax the colonies, and made war against them before the colonies declared that because of England making war, they were of right independent.