The Missouri Compromise is transparently the catalyst for all other issues that had risen. In 1820, Washington decided on “a bundle of three compromises” led by Henry Clay. The primary of the three was that no longer could slave states be addmitted into the Union above the 36°30' latitude line. The line is Missouri's southern border. The other two compromises are basically the terms of the 36°30' rule: No slave state above, and only slave states below it. Neither North nor South was completely satisfied; nevertheless, they both gained some. It was a weak beginning to abolition, and it did ease slight tension. On the other hand, the extremists on either side were definitely not happy because one side had not truly lost. The institution of slavery was necessary and practically sacred to Southerners. The land in the Southwest was also viewed by many plantation owners as unusable because the soil wasn't good for cotton. It was a gain ultimately for the North, for it limited the spread of slavery to any newly acquired territories(from the Mexican War and Oregon Territories). Some political compromise was made, but it was brittle in that it would fall to pieces in the next 34 years.
Along with a bill that would hopefully end the Mexican War, the Wilmot Proviso added something to the table. Under James K. Polk and at the near end of the Mexican War, David Wilmot put his word in that footing a bill to bring peace is not the only thing that would help. It stated that in the lands from the Mexican territory none of it would succumb to slavery. This went against the Missouri Compromise and was an attempt to limit slavery even further. It never actually became a federal law; however, it was suppoerted and endorsed by almost all of the free states. If the Proviso had passed, it would have extirpated popular sovereignty. David Wilmot, proposer of the amendment to the bill, also single handedly increased sectional tensions between North and South because the two had opposing views on the matter. Government had little options to stop him from proposing this, which makes it less Washington's fault, but they still failed to compromise politically. This was the first crack at the Missouri Compromise that led to it's decent.
Eighteen years after the Wilmot Proviso proposal was declined, even further were the sectionalist tensions increased by the Kansas-Nebraska Act of 1854. By that time, Pierce had been elected president from the Democratic Party and his Expanionist plan was in full swing. Nicaragua, Asia, and Cuba had all been settled with the Clayton-Bulwer Treaty in '50, Gadsden Purchase in '53, and Ostend Manifesto in '54. Stephen Douglass had taken the stage to settle the slavery issue in the Kansas-Nebraska Territory, so he decided to try and split them. Kansas would be a slave state and Nebraska would be a free one. The North was shocked by Douglass's attempt to ease it into Congress. By itself, the bill caused the repeal of the Missouri Compromise and completely destroyed political compromise with slavery. Sectionalist tensions were as high as ever, and the repeal became somewhat of an instigator for issues later to come. Southerners wanting slavery in Kansas were appauled by the North allegedly trying to control Kansas. The Republican Party was formed because of the Act and became the nation's number 2 political party almost overnight. This contributed to the differences between North and South, and it effecticely wrecked both the 1820 and 1850 compromises. But this was not the Union's breaking point.
The election of 1860 was not the final straw either, but it came dangerously close to the point of the South threatening to secede. Abraham Lincoln, Stephen Douglass, John C. Breckinridge, and John Bell were the four candidates of the election. Lincoln had two years earlier beaten Douglass to the Senatorial seat, and now he tried to beat him again with the Presidential Election. The Democratic Party was split, so they decided on two cadidates which were Breckinridge and Douglass. The Constitutional Union chose Bell, and the Republicans had Lincoln. With his campaign revolving around westward expansion and abolition, Lincoln did not get very many supporters. The people of the country more so wanted Douglass. With only 40% of the entire country's votes Lincoln won the presidency. Douglass came in second with ~30%, Breckinridge ended third with ~20%, and Bell rounded out the four in las place with only about 13%. Lincoln, the sectional president, had not been elected for four days when South Carolina seceded. The blame rested on Buchanan's shoulders, for he was the president still until March 1861. This was ultimately the cause for disunion. During the election, or rather the time of campaigning, South Carolina had stated that if Lincoln was elected they would secede; and they went through with their word. Sectionalist tensions has risen to a peak and cracked under pressure. The Missouri Compromise was no longer available to be a scapegoat, and neither were the other political controversies. The Union's last leg to stand on was kicked out from under it. It would take several more months until the actual war started; however, by this point it is clearly inevitable. Political compromise was out of the question, and by then it was only a matter of time before the first shots were fired.