10.3 The political conflicts of the 1850s, driven largely by the unresolved issue of slavery, set the stage for the American Civil War. The expansion of U.S. territory west of the Mississippi, especially after the Mexican War, reignited debates over whether new states and territories would permit slavery, threatening the balance of power in Congress. Efforts like the Missouri Compromise of 1820 temporarily maintained peace by limiting slavery north of a certain latitude, but the acquisition of new lands challenged this arrangement. The Compromise of 1850, proposed by Henry Clay, attempted to balance interests by admitting California as a free state and allowing popular sovereignty in other territories, but it failed to resolve tensions and included a controversial Fugitive Slave Act. Political parties like the Whigs declined, while new movements such as the nativist Know-Nothings and the antislavery Republican Party emerged. The passage of the Kansas-Nebraska Act in 1854, which repealed the Missouri Compromise and allowed territories to decide on slavery by popular vote, further inflamed sectional divisions, leading to increased conflict and the eventual breakdown of national unity.