What was the Wilmot Proviso
The Wilmot proviso was an amendment offered to an appropriations bill in 1846 by David Wilmot, a freshman senator from Pennsylvania. The United States had just gained substantial territory as a result of its victory in the Mexican War and Wilmot, obviously idealistic, proposed that the provisions of the Northwest Ordinance which prohibited slavery be extended to all territories gained from Mexico. In a highly inflammatory speech on the floor of the House, he said
God forbid that we should be the means of planting this institution [slavery] upon it.
Wilmot's proviso never became law, but did fuel the slavery debate which most Congressmen believed had been at least temporarily resolved by the Missouri Compromise. Sen John C. Calhoun of South Carolina was so incensed that he offered the "Calhoun Resolutions" in the Senate, which argued that the Fifth Amendment's guarantee against taking property without just compensation prohibited Congress from interfering with slavery, as slaves were property.
Calhoun's resolutions also did not become law but they with the Wilmot proviso polarized the slavery debate and solidified the arguments on each side. Senator Henry Clay said that the two arguments were like a pair of scissors: One alone could do nothing, but together they could cut the country in two.
Something interesting to add to this answer is that Wilmot was not necessarily "idealistic" in his attempt to keep slavery out of the territories, at least not in the sense that he was engaged in a moral crusade against slavery. He was mainly opposed to limiting the expansion of slavery because he, and many other Free Soilers, wanted to preserve what they saw as "white man's territory." He assured others that his stance did not come from a "squeamish sensitiveness..." for slave, and that his only purpose was to
...preserve for free white labor a fair country...where the sons of toil, of my own race and my own color can live without the disgrace which association with negro slavery brings upon free labor.
He was opposed to the idea that white men should have to compete with black labor in any form. Wilmot had also been a supporter of the James K. Polk and, initially, the Mexican War. His involvement in the sectional dispute that followed the Mexican War is a great example of the nuances and (from a modern perspective) contradictions that characterized slavery as a political issue.
The war with Mexico (1846–48) helped to inflame sectional tensions between peoples of the Southwest and Northern territories. The Wilmot Proviso, although voted down in the Senate, alienated slaveholders by barring slavery from any territorial acquisition as a result of the Mexican War. Land acquired from Mexico thus became the focus of hot debate, and this led to the The Free Soil Party running a candidate for president in 1848. The Wilmot Proviso helped to incite differences people peoples of different backgrounds and helped to further tension which led to the Civil War in 1861. It was not an easy road for reoncilation between the slave states and the free states.
The wilmot proviso was a document proposed by David Wilmot, who feared the spread of slavery, asking for no slavery in the territories acquired from the Mexican War. The bill was reviewed but never officially put into effect by congress. The document caused a stir even more in the slavery debate.
Wilmot proviso was specifically provided for the prohibition of slavery in lands acquired from Mexico in the Mexican war. Congress did not pass the Wilmot Proviso. This was one of the causes of the Civil War.