There was great tension between pro-slavery and anti-slavery representatives over how new territories won would handle the issue of slavery.
The Mexican-American War and the 1848 Treaty of Guadalupe Hidalgo, forced onto the remnant Mexican government, drew some criticism in the U.S. for their casualties, monetary cost, and heavy-handedness. Furthermore, the question of how to treat the new acquisitions also intensified the debate over slavery and in many ways inflamed it, as potential westward expansion of the institution took an increasingly central and heated theme in national debates preceding the American Civil War.