Wilmot Proviso
It was widely, accurately believed that war with Mexico would be won with large territorial gains. The Wilmot Proviso was the first congressional attempt to address slavery in these projected new territories, by proposing to ban it. Congress rejected the Wilmot Proviso, but not quickly or smoothly. Protracted debate aggravated sectional tensions. The repeated failure of Congress, and later also the President and Supreme Court, over the next decade to resolve the issue of slavery in the territories was a major cause of the Civil War.