Summary

In 1803, Thomas Jefferson appointed Meriwether Lewis to organize an expedition into the Louisiana Territory to explore and map the area but also to find an all-water route from the Missouri River to the Pacific Coast. The Louisiana Purchase and the journey of Lewis and Clark’s Corps of Discovery captured the imagination of many, who dedicated themselves to the economic exploitation of the western lands and the expansion of American influence and power. In the South, the Adams-Onís treaty legally secured Florida for the United States, though it did nothing to end the resistance of the Seminoles against American expansionists. At the same time, the treaty frustrated those Americans who considered Texas a part of the Louisiana Purchase. Taking matters into their own hands, some American settlers tried to take Texas by force.

The Missouri Crisis created a division over slavery that profoundly and ominously shaped sectional identities and rivalries as never before. Conflict over the uneasy balance between slave and free states in Congress came to a head when Missouri petitioned to join the Union as a slave state in 1819, and the debate broadened from simple issues of representation to a critique of the morality of slavery. The debates also raised the specter of disunion and civil war, leading many, including Thomas Jefferson, to fear for the future of the republic. Under the Missouri Compromise, Missouri and Maine entered the Union at the same time, Maine as a free state, Missouri as a slave state, and a line was drawn across the remainder of the Louisiana territory north of which slavery was forbidden.

The establishment of the Lone Star Republic formed a new chapter in the history of U.S. westward expansion. In contrast to the addition of the Louisiana Territory through diplomacy with France, Americans in Texas employed violence against Mexico to achieve their goals. Orchestrated largely by slaveholders, the acquisition of Texas appeared the next logical step in creating an American empire that included slavery. Nonetheless, with the Missouri Crisis in mind, the United States refused the Texans’ request to enter the United States as a slave state in 1836. Instead, Texas formed an independent republic where slavery was legal. But American settlers there continued to press for more land. The strained relationship between expansionists in Texas and Mexico in the early 1840s hinted of things to come.

President James K. Polk’s administration was a period of intensive expansion for the United States. After overseeing the final details regarding the annexation of Texas from Mexico, Polk negotiated a peaceful settlement with Great Britain regarding ownership of the Oregon Country, which brought the United States what are now the states of Washington and Oregon. The acquisition of additional lands from Mexico, a country many in the United States perceived as weak and inferior, was not so bloodless. The Mexican Cession added nearly half of Mexico’s territory to the United States, including New Mexico and California, and established the U.S.-Mexico border at the Rio Grande. The California Gold Rush rapidly expanded the population of the new territory, but also prompted concerns over immigration, especially from China, and led to a massive loss of Native American life in the state.

The acquisition of lands from Mexico in 1848 reawakened debates regarding slavery. The suggestion that slavery be barred from the Mexican Cession caused rancorous debate between North and South and split the Democratic Party when many northern members left to create the Free-Soil Party. Although the Compromise of 1850 resolved the question of whether slavery would be allowed in the new territories, the solution pleased no one. The peace brought by the compromise was short-lived, and the debate over slavery continued.