Wilmot Proviso Is Introduced
The Wilmot Proviso was introduced on August 8, 1846, in the United States House of Representatives as a rider on a $2 million appropriations bill intended for the final negotiations to resolve the Mexican-American War.
The intent of the proviso, submitted by Democratic Congressman David Wilmot, was to prevent the introduction of slavery in any territory acquired from Mexico. The proviso did not pass in that session or in any other session when it was re-introduced over the course of the next several years, but many consider it as one of the first events on the long slide to secession and Civil War which would accelerate through the 1850s.
Former President John Quincy Adams also expressed his belief that the war was fundamentally an effort to expand slavery in a speech he gave before the House on May 25, 1846. In response to such concerns, Democratic Congressman David Wilmot introduced the Wilmot Proviso, which aimed to prohibit slavery in any new territory acquired from Mexico. Wilmot's proposal did not pass Congress, but it spurred further hostility between the factions.
Wikipedia: Mexican-American War
Wikipedia: David Wilmot