Wilmot Proviso
In March 1846, a skirmish broke out on the northern side of the Rio Grande, ending in the death or capture of dozens of American soldiers. Shortly thereafter, the United States declared war on Mexico, beginning the Mexican–American War. In August 1846, Polk asked Congress for an appropriation that he hoped to use as a down payment for the purchase of California in a treaty with Mexico, igniting a debate over the status of future territories. A freshman Democratic Congressman, David Wilmot of Pennsylvania, offered an amendment known as the Wilmot Proviso that would ban slavery in any newly acquired lands. The Wilmot Proviso was defeated in the Senate, but it injected the slavery debate into national politics.