There is no provision for secession in the U.S. Constitution.
Threats to secede from the Union had arisen for decades, and during the Nullification Crisis three decades earlier it appeared that South Carolina might try to break away from the Union.
South Carolina Was the First State to Secede
Following the election of Abraham Lincon, southern states began to make more serious threats to secede.
The first state to secede was South Carolina, which passed an "Ordinance of Secession" on December 20, 1860. The document was brief, essentially a paragraph which stated that South Carolina was leaving the Union.
Four days later, South Carolina issued a “Declaration of the Immediate Causes Which Justified the Secession of South Carolina from the Union.”
South Carolina's declaration made it abundantly clear that the reason for secession was the desire to preserve slavery.
South Carolina’s declaration noted that a number of states wouldn't fully enforce fugitive slave laws; that a number of states had “denounced as sinful the institution of slavery”; and that “societies,” meaning abolitionist groups, had been allowed to operate openly in many states.
The declaration from South Carolina also referred specifically to the election of Abraham Lincoln, stating that his "opinions and purposes are hostile to slavery."
Other Slave States Followed South Carolina
After South Carolina seceded, other states also broke from the Union, including Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas in January 1861; Virginia in April 1861; and Arkansas, Tennessee, and North Carolina in May 1861. Missouri and Kentucky were also considered to be part of the Confederate States of America, though they never issued documents of secession.