The American Civil war began with a Confederate attack on Fort Sumter. At the same time it was never established that you can opt out of the US, that’s generally not how countries work.

The Confederacy would not have happened if it wasn’t for fears of abolishing slavery.

Lincoln’s election provoked South Carolina’s legislature to call a state convention to consider secession. South Carolina had done more than any other state to advance the notion that a state had the right to nullify federal laws and even secede. On December 20, 1860, the convention unanimously voted to secede and adopted a secession declaration. It argued for states’ rights for slave owners but complained about states’ rights in the North in the form of resistance to the federal Fugitive Slave Act, claiming that Northern states were not fulfilling their obligations to assist in the return of fugitive slaves.

-Wikipedia

It was “states rights for me but not for thee”.