Final answer:
The Mexican American War was primarily caused by American expansionist ambitions and greed, which strained foreign relations with Latin America and reignited the slavery debate in the United States.
Step-by-step explanation:
The Underlying Cause of the Mexican American War
The primary underlying cause of the Mexican American War was American expansionism fueled by greed and the desire for territorial gains. This war exacerbated foreign relations in Latin America, as Mexico and other Latin American countries believed the United States deliberately provoked the conflict. Moreover, it intensified "Yankeephobia" throughout the region, fostering distrust and suspicion towards the United States. The war disturbed the political equilibrium concerning slavery, igniting a renewed debate over its expansion, especially with the looming possibility of new slaveholding territories being integrated into the Union. This concern was encapsulated by the Wilmot Proviso, an attempt to ban slavery in territories acquired from Mexico, which heightened sectional tensions though it ultimately failed to become law.