The best known example of secession in American politics was that of the Confederate States of America from the Union in 1860 and 1861. However, the concept was not new and had been proposed several times in the past for a variety of reasons.
During the War of 1812, New England felt that the war's objectives served only the interests of the South and West and was being conducted at their expense. The Hartford Convention of 1814 brought together dissatisfied representatives from the region, who met in secret to discuss their strategy. One of the options considered as secession of New England from the United States. When the war came to a quick end, the motivation for secession evaporated.
In the 1840s, abolitionists seriously considered the possibility that the North should secede, in order to free itself from the immoral Constitution of the United States, which gave permanent legal support to slavery in America. In 1843, the Massachusetts Anti-Slavery Society endorsed disunion by a vote of 59 to 21.
Just before the outbreak of hostilities in 1861, Fernando Wood, the mayor of New York City, proposed the the city should secede from New York State and become its own "free port." Investments by New Yorkers in the South along with profitable commercial relations made the city reluctant to support the Civil War, but Wood's radical suggestion was not followed.
The first state to secede was Mississippi in December 1860. It was followed by five more in January 1861 and Texas on February 1, bringing the total to seven. These seven met in Montgomery and formed the CSA in that month. Later, they were joined by four more, with North Carolina being the last in late May.
During the war, the western part of Virginia seceded from the state and became West Virginia in 1863. The legality of this was questioned, since Virginia was one of the founding states and there was no provision in the Constitution for a portion of a state to secede without at least the approval of the remainder.
A similar situation developed after the end of the war. The question was, since secession was unconstitutional, had the states ever legally seceded? If not, had they ever ceased to have full statehood in the Union?
This issue eventually was settled by considering the constitution of each state to be the most important factor. The act of secession, being unconstitutional at the federal level, annihilated the state constitution in the federal eyes. After that, until acceptable constitutions were developed, the federal government was freed from many of the usual restrictions. This was the theoretical basis for Reconstruction.
There was, of course, no written basis for this interpretation since the writers of the Constitution had never contemplated secession. The Civil War permanently ended serious discussion of secession except occasionally in Texas.