After years of simmering tension, disagreements between the northern and southern states erupted as a result of the Mexican-American War (1846–1848). The acquisition of thousands of miles of western territory from Mexico raised questions about the expansion of slavery. The controversy grew especially intense in the U.S. Congress, where antislavery and proslavery factions used increasingly hostile language and threatened to break up the Union. Undertaking the difficult challenge of calming the situation was Senator Henry Clay, a veteran Kentucky legislator and three-time presidential nominee. Then in the twilight of his career, Clay used his legendary political skills to persuade both northern and southern states to make concessions in order to avoid civil war.