Short answers, pulled from the story.
The Three-Fifths Clause counted enslaved people as three-fifths of a person for congressional representation, and Article I, Section 9 allowed Congress to ban the importation of slaves only after the 1st of January 1808. These provisions gave Southern states extra political power while creating a twenty-year moratorium on ending the slave trade.
The Missouri Compromise admitted Maine as a free state while allowing Missouri to enter as a slave state and drew a geographic line across the Louisiana Purchase territory at parallel 36°30′ north. This arrangement banned slavery in lands north of this boundary except within Missouri itself and quieted sectional tensions for three decades until the Kansas-Nebraska Act repealed it in 1854.
South Carolina opposed the Tariff of Abominations passed by Congress in 1828 so strongly that they began organizing state politics around the tariff issue alone. Vice President John C. Calhoun developed the constitutional theory of state nullification through his 1828 South Carolina Exposition and Protest to challenge federal authority over economic policy.
The House passed the Pinckney Resolutions establishing what became known as the gag rule, which prohibited even receiving anti-slavery petitions with votes of one hundred seventeen to sixty-eight. Former President John Quincy Adams led opposition to these restrictions arguing they violated First Amendment rights to petition government for redress of grievances.
Senator Stephen A. Douglas introduced legislation that repealed the Missouri Compromise line at 36°30′ north and established popular sovereignty allowing settlers in each territory to decide whether to permit slavery through local vote. This doctrine declared that Congress had no authority to impose slavery or ban it within territories created by federal law.
The conflict began the 12th of April 1861 when Confederate forces bombarded Fort Sumter in Charleston harbor, South Carolina. Seven slave states of the Deep South formed the Confederate States of America after Abraham Lincoln was elected in November 1860 but before he took office in March 1861.