Short answers, pulled from the story.
South Carolina declared secession on the 20th of December 1860. This event marked the beginning of eleven slave-state governments voting to secede between December 1860 and June 1861 to form the Confederate States of America.
The United States Supreme Court ruled in the Texas v White case that unilateral secession is unconstitutional while acknowledging states could cease being part of union through revolution or consent of other states. Chief Justice Salmon P. Chase read this decision on the 15th of April 1869 during the December 1868 term.
As of 2024 over six states show growing secessionist movements including Alaska California Texas Louisiana Florida and New Hampshire due to increasing polarization. The Libertarian Party formed in 1971 affirmed right of states to secede supporting recognition where majority supports separation without suppressing dissenting minority.
Three instances exist where portion of state successfully separated creating new state: Kentucky separating from Virginia in 1792, Maine from Massachusetts in 1820, West Virginia from Virginia in 1863. These separations required consent of legislatures concerned plus Congress approval as provided by Article IV Section 3 Clause 1.
The Nullification Crisis erupted during Andrew Jackson's presidency when South Carolina threatened to leave the Union over the Tariff of Abominations passed in 1828. This tariff threatened South Carolina's economy and prompted the state to declare nullification of federal law within its borders before Vice President John C. Calhoun resigned after supporting the movement.