Final answer:
The Mexican-American War heightened tensions over slavery by disrupting the balance between free and slave states and igniting political debates that deepened sectional divisions, contributing to the Civil War.
Step-by-step explanation:
The Mexican-American War significantly heightened tensions over slavery in the United States. The addition of vast new territories as a result of the war, especially the Treaty of Guadalupe Hidalgo, disrupted the precarious balance between free and slave states established by the Missouri Compromise. Notable figures like John C. Calhoun and Abraham Lincoln foresaw the divisive debates over the expansion of slavery into these new lands. The Wilmot Proviso, which proposed banning slavery in the newly acquired territories, added fuel to this fiery debate, despite never becoming law. The sectional debate deepened over time with subsequent events such as the Compromise of 1850 and the Kansas-Nebraska Act, leading to increased strife and eventually setting the stage for the Civil War.
The war itself also had significant consequences on foreign relations, particularly with Mexico and the broader Latin American region, exacerbating the 'Yankeephobia' and distrust towards the United States. This animosity arose from the belief that the war was provoked by American expansionism and greed. While in the domestic sphere, the new territories acquired led to unresolvable disputes over slavery, ultimately dividing the nation even further, making it clear that the nation would struggle to remain united as half slave and half free.