Because the fort informed them that it would be firing upon them when those ships arrived. The fort was offered a truce, but the Fort declined.
Look, virtually all of Lincoln's cabinet told him his plan would start a war. The Commander of the Fort, Major Anderson, wrote that this plan was going to start a war.
Lincoln did in fact start the war by ordering those warships into Charleston. The war started April 5th, not April 11th, and Lincoln started it.
Now you should also know that the reason the slavocracy rebelled is because of slavery.
I have maintained all along that people wanting independence don't have to justify why they wish to exercise the right to independence. It is the people who attack them that has to justify why they are attacking people who just want out from under their control.
Only the Union's reasons for launching that war matter. And their reason was NOT slavery.
I know you’ve said you don’t care why people want to rebel. So I assume if Minnesota became majority Muslim and they declared they were an independent country under sharia law, passed laws making female genital mutilation mandatory, allowing child brides, murder for the crime of homosexuality etc. You’d be one of the first ones to support them?