New debates over slavery that emerged as a result of the Mexican-American War set the stage for the Great Compromise of 1850, which was negotiated at the war’s end and attempted to resolve the slavery debate. The fifteen years following the war were filled with disputes concerning slavery, especially as it pertained to new territory gained from the war that was soon entering the union. The Mexican-American War marked a turning point in the debate over slavery in the United States because it reopened the slavery debate in Congress due to the disputed motives of the war, the failure of the Wilmot Proviso, and the size of the territory included in the treaty of Guadalupe Hidalgo.

The anti-war sentiment surrounding the Mexican-American War was led by anti-slavery citizens who questioned the motives behind the war, causing heated debates on the topic of slavery to not only revive in Congress but also remain there for over a century due to opposing reactions from the North and the South. Broadly speaking, Democrats in the West and South firmly supported the war. However, Whigs in the Northeast, specifically in places like New England, strongly opposed the war. These Whigs believed the South would demand any new territory gained from the war enter as non-free states, causing the North to assume the incentives behind fighting the war were based on attempts to increase slavery. These differing viewpoints caused a sharp turn in America’s posture towards slavery, as Congressional fights commenced yet again on this topic. The strong opposition the Whigs had, though, ensured the debates in Congress would not subside until a sustainable compromise was agreed upon, which is different from previous congressional debates over slavery.

The failure of the Wilmot Proviso only heightened tensions, as it increased the pressure on the legislative branch to reach an acceptable compromise to an unprecedented level. Attempting to more clearly articulate the role of slavery in new territory gained from the war, David Wilmot, proposed an amendment to the army appropriations bill that became known as the Wilmot Proviso. This proviso stated, “neither slavery nor involuntary servitude shall ever exist in any part of the said territory.” If this proviso had passed Congress, it would have ensured all the new territories obtained from Mexico would have entered the union as free states. However, it faced opposition in the Senate because of southern influence and ultimately it was defeated. This resulted in the debate over slavery continuing internally within Congress as ultimately no resolution had been reached.

The feared new territory became a reality as a result of the treaty of Guadalupe Hidalgo, forcing quick decisions over the status of this territory, in regards to slavery, as citizens began to demand answers. James K. Polk, the president of the United States at this time, heightened the slavery debate through his dispute with Nicholas Trist over the terms that should be negotiated between the United States and Mexico at the end of the war. The deal, known as the treaty of Guadalupe Hidalgo, which guaranteed the United States a substantial amount of land in return for a payment to Mexico of fifteen million dollars, was accepted and signed by Trist despite extreme opposition from Polk. The approval of this treaty by Congress further escalated tensions surrounding slavery as the treaty did not clearly address the role that slavery would play in these new territories. Polk attempted to convince Americans that they should not worry about the new territory being comprised of slave states. This statement, though, was controversial for Polk to be making, since the southerners, who wanted slavery, were also the ones supporting the war effort the most. Therefore, Polk had to strike a careful balance to ensure he did not face overwhelming opposition from either the North or the South. This enormous controversy established a gateway through which the Great Compromise was able to be negotiated and agreed upon post the conclusion of the Mexican-American War.

Despite the Great Compromise being established nearly two years after the conclusion of the Mexican-American War, it was mainly influenced by the impacts the war had on the public’s perception of the importance of resolving the slavery debate in a timely fashion. The dividing line drawn between the North’s and South’s opinions surrounding this war took the once nearly silenced issue of slavery and revived in the public sphere. Quick decisions made during the war, especially by people like Trist, created an unparalleled environment for the slavery debate to occur in. These effects in the war created a new atmosphere in the United States that provided a foundation for the Great Compromise of 1850 to be established on.



Battle of Monterrey, September 1846, Mexican-American War, Mexico, 19th century. Photograph. Britannica ImageQuest, Encyclopædia Britannica, 25 May 2016.
Murrin, John M., et al. Liberty, Equality, Power: A History of the American People. Boston: The Thomas Corporation, 2008. Print.