How did the Mexican war and Westward expansion bring apart the southern secession?

please help me. i have an ap history final tomorrow and its one big dbq (essay) and its covering from the 1840's to the civil war. Some of the documents are Missouri compromise, treaty of Guadalupe hidalgo, a discourse of the constitution by John C. Calhoun, ostend manifesto, know nothing platform, southern view of Kansas, the dred scott decision, a gone with the wind ad, Lincoln's cooper union speech, the south carolina ordinance, the crittenden compromise, and union and confederate resources in 1860