Passage: Regional conflict was most pronounced over the issue of slavery. One example of a provision repeatedly supported by the House but blocked by the Senate was the Wilmot Proviso, which sought to ban slavery in the land gained during the Mexican–American War. Conflict over slavery and other issues persisted until the Civil War (1861–1865), which began soon after several southern states attempted to secede from the Union. The war culminated in the South's defeat and in the abolition of slavery. All southern senators except Andrew Johnson resigned their seats at the beginning of the war, and therefore the Senate did not hold the balance of power between North and South during the war.

Question: What did the Wilmot Proviso attempt to make illegal in certain areas?
ban slavery