Short answers, pulled from the story.
An Ordinance of Secession is the name given to multiple resolutions drafted and ratified in 1860 and 1861 by which each seceding Southern state formally declared secession from the United States. South Carolina, Mississippi, Georgia, and Texas also issued separate documents purporting to justify secession. In total, eleven Southern states and one territory ratified such ordinances.
South Carolina was the first state to secede, ratifying its Ordinance of Secession on the 20th of December 1860. The state's action came in the wake of Abraham Lincoln's election to the presidency. Six more Deep South states followed within weeks.
In his State of the Union Address on the 3rd of December 1860, President James Buchanan stated that the Union rested only upon public opinion and that conciliation was its only legitimate means of preservation. His position contrasted sharply with Lincoln's view that preserving the Union by force was both justified and necessary.
In Missouri, the state government called a convention whose members disfavored secession, and Union military intervention quickly restored Union control. In Kentucky, both the legislature and public opinion firmly opposed secession; only a rump convention with little influence purported to secede, and when Confederate armies invaded in 1862, local recruitment proved weak and Union forces soon defeated the invasion.
The first seven seceding states were motivated mainly by the 1860 election of Abraham Lincoln, who had very little support among Southern voters, and the direct threat to slavery his election posed. The four states further north shared those motivations but also cited the federal policy of using military force to preserve the Union.
Tennessee rejoined the Union in 1866, one year after the Civil War ended in 1865, making it the first former Confederate state readmitted during Reconstruction. Tennessee's path had been contested: voters initially rejected a secession convention in February 1861, before approving secession in June 1861 after Lincoln called for troops following the attack on Fort Sumter.