The early nineteenth century saw the young republic dramatically expand its territory and wrestle with the consequences of growth. Thomas Jefferson orchestrated the 1803 Louisiana Purchase, acquiring roughly 828,000 square miles from France for $15 million and doubling the nation's size, then commissioning the Lewis and Clark Expedition (1804 to 1806) to map routes to the Pacific with the aid of Sacagawea. The War of 1812 against Britain over trade restrictions and impressment ended in a stalemate but ignited a surge of American nationalism. Abroad, the 1823 Monroe Doctrine declared the Western Hemisphere closed to further European colonization, establishing a cornerstone of U.S. foreign policy for the next century.
At home, the belief in Manifest Destiny, the idea that American expansion across the continent was both justified and inevitable, drove westward migration, the annexation of Texas, and the Mexican-American War (1846 to 1848), which added California, Nevada, Utah, and parts of other states to the Union. The 1848 discovery of gold at Sutter's Mill sparked the California Gold Rush, accelerating statehood and westward migration. Federal policies like the 1862 Homestead Act, which granted 160 acres to settlers who farmed the land for five years, and the completion of the Transcontinental Railroad in 1869 further fueled settlement of the Great Plains, though both developments came at the expense of Native American communities displaced from their ancestral lands. President Andrew Jackson's Indian Removal Act of 1830 culminated in the Trail of Tears (1838 to 1839), in which the Cherokee and other nations were forced from their southeastern homelands to Indian Territory, with thousands perishing along the march.
This territorial expansion intensified the sectional crisis over slavery. To preserve the balance between free and slave states, Congress passed the Missouri Compromise of 1820, admitting Missouri as a slave state and Maine as a free state while prohibiting slavery north of the 36°30′ line in the Louisiana Territory. The Compromise of 1850 admitted California as a free state, established popular sovereignty in other new territories, and enacted a stricter Fugitive Slave Law. The 1857 Dred Scott decision, in which the Supreme Court ruled that African Americans were not citizens and that Congress could not prohibit slavery in territories, inflamed sectional tensions and hastened the march toward civil war. Meanwhile, successive waves of immigration, from Irish and Germans in the mid-nineteenth century to Southern and Eastern Europeans around the turn of the twentieth century and beyond, continually reshaped the nation's culture, economy, and identity.