Western expansion had accelerated greatly by the mid-1800s. The settlement of the West caused even more complications in issues like slavery and the rights of non-white people. Western migration was fueled by the popular philosophy of Manifest Destiny, which held that white Americans must settle in all the new territory to protect it from people like Mexicans and Indigenous Americans, whom they saw as dangerous, uncivilized, and unsuitable for American citizenship. While many settlers likely believed in Manifest Destiny, the primary motivation for western pioneers was primarily economic opportunity and the chance to live life outside the confines of eastern civilization. The California Gold Rush of the 1840s in particular drew thousands of prospectors seeking fortunes that few found. The mid-19th century West largely governed itself, but the issue of slavery hung over the West. As populations grew, new industries demanded laborers—and the 1820 Missouri Compromise had defined the legality of slavery only as far as modern-day Oklahoma.

Many in the North saw the West as an opportunity to expand the population—and therefore the power—of a slave-free America. Others, including President James Polk, thought the best solution was simply to draw the Missouri Compromise line further west and continue the North-South divide of slave states and free states.