Final answer:
The primary beneficiaries of the Mexican-American War were pro-slavery proponents in the southern U.S., as the war resulted in new territories that could potentially allow for the expansion of slavery. Key legislative actions included the debated Wilmot Proviso and the Compromise of 1850 which attempted to address the tensions arising from these territorial gains.
Step-by-step explanation:
The group that primarily benefitted from the war with Mexico were the pro-slavery advocates in the United States, particularly in the southern states. They gained extensive new territories in which they hoped to expand the institution of slavery. The legislation that sparked considerable debate during this era was the Wilmot Proviso, which proposed the exclusion of slavery in any territory acquired from Mexico.
This debate led to the creation and passage of the Compromise of 1850, a series of measures introduced by Senator Henry Clay to resolve the tension between free and slave states in the new territories. Another significant legislative response was the Fugitive Slave Law, which exacerbated tensions between the North and South by requiring the return of escaped slaves and penalizing those who aided in their escape.