Even though the Wilmot Proviso was defeated, it was significant. Why?

The Mexican War produced the first powerful antiwar movement in American history. Many opponents of the war, like opponents of Texas's annexation, believed that the war was primarily a means of spreading slavery into the West. Also, they thought that it was wrong for the United States to wage a war to conquer another nation's lands. On the other hand, some opponents of the war had very different motives. They opposed statehood for Texas and other lands in the Southwest because many of the inhabitants of these lands were Mexicans and Indians. They did not want these people to become U.S. citizens and voters. So some Americans opposed the war because they detested slavery, while others opposed it because they did not want non-whites to become American citizens.

Writer Henry David Thoreau strongly opposed the war because he considered it a war for slavery. He protested against it by refusing to pay his taxes in the state of Massachusetts. While in jail for defying the law, Thoreau wrote an influential essay, Civil Disobedience. In this work, he argued that citizens had a right, even a duty, to resist laws and government actions that they considered terribly unjust.

Also in 1846, Pennsylvania congressman David Wilmot introduced a measure in Congress, the Wilmot Proviso, that stated that slavery would be prohibited in any land gained from Mexico during the war. (Wilmot was opposed to the spread of slavery, but also wanted to ensure that the West would be settled by white Americans, and his proposal was sometimes referred to as the "White Man's Proviso.") Congress defeated the Wilmot Proviso. Ominously, the vote on the Wilmot Proviso was not along party lines, but along the Mason-Dixon line. Northerners voted for the Proviso, while Southerners voted against it. This vote foreshadowed the division of the nation during the Civil War.