In the mid-1800s, Americans surged westward past the Mississippi River, the previously drawn boundary of the frontier. As settlers migrated toward the Pacific coast in their overloaded wagons, the West became the fastest growing area of the country. Despite fierce resistance from Native Americans, Mexicans, and the British, Americans eventually claimed the entire region west of the Mississippi. However, westward expansion had its costs: settlers, Native Americans, and the integrity of the Union suffered at its hands.

Fueling the expansion westward was the popular belief that it was America’s manifest destiny to expand across Texas, toward the Pacific coast. In 1845, a New York journalist wrote of “our manifest destiny to overspread and to possess the whole of our continent which Providence has given us for the development of the great experiment of liberty.” Manifest destiny tapped into America’s nationalist spirit, which had been growing since the War of 1812, and echoed Protestant beliefs that America was a “called nation”—that is, chosen by God as a haven where Protestants could spread their faith.

During the 1820s, Americans streamed into Texas, then a Mexican territory, often receiving land grants from the Mexican government. Mexico gave these grants in the hopes of promoting the region’s trade and development. By 1830, about 7,000 Americans lived in Texas, outnumbering Hispanic settlers two to one and alarming the Mexican government. In 1834, rebel Texan leaders, most of them American, declared their independence from the Mexican dictatorship. After two years of fighting, Texas became an independent republic, although the Mexican government refused to officially acknowledge its independence.

Because most Texan settlers were American, the question immediately arose of Texas’s potential statehood. President John Tyler, who became president in 1841 after William Henry Harrison died in office, favored the annexation of Texas and its admission to the Union. In 1844, Democrat James K. Polk won the presidential election on a platform determined to “re-annex Texas and re-occupy Oregon.” One month into his presidency, Congress voted to annex Texas. In 1845, Texas was admitted into the Union as the twenty-eighth state. Mexico, still refusing to recognize Texas’ independence, threatened war over the annexation.

War erupted a year later over the new state’s borders: the U.S. argued that the southern Texas border lay along the Rio Grande River, while Mexico insisted that the border lay much farther north. After trying unsuccessfully to buy the New Mexican and Californian territories from Mexico, the U.S. found a pretense to declare war against Mexico in 1846, when Mexican troops crossed the Rio Grande. The Mexican War spread throughout Texas, New Mexico, and California, and into the Mexican interior, finally ending in U.S. victory. With the Treaty of Guadalupe Hidalgo, signed in February 1848, Mexico ceded Texas, New Mexico, and California to the U.S. for $15 million. (Note that this ceded territory encompassed present-day Arizona, Nevada, California, Utah, and parts of New Mexico, Colorado, and Wyoming.) The treaty secured the West for American settlement, and American land now stretched continuously from the Atlantic to the Pacific Ocean.

Polk’s presidential campaign slogan, “Fifty-four Forty or Fight,” referred to the latitude coordinates of northwest territory claimed by both the U.S. and Great Britain. The area included present-day Oregon, Washington, and Idaho; parts of Montana and Wyoming; and much of western Canada. Northerners also pushed for acquisition, since the admission of Oregon, a free state, would balance the annexation of slave-holding Texas. However, Polk, once in office, could not commit to “fight” for the territory—already caught up in border disputes with Mexico, he did not wish to engage in further conflict and instead proposed a compromise with Britain. The 1846 compromise divided the Oregon territory along the forty-ninth parallel. South of this line lay U.S.-owned Oregon, and north lay the British-owned Washington territories. Oregon was admitted as a state in 1859.

In January 1848, an American carpenter struck gold in California’s Sierra Nevada Mountains. As news of this discovery drifted east, a gold rush began, drawing hordes to the West Coast in search of their fortunes. California attracted about 100,000 immigrants in a single year, including Mexicans, Europeans, and Americans from the East coast. This influx of settlers led to the growth of numerous cities and mining towns, and pressure grew for California to organize its own government, either independent of the Union or as a state.

A central aspect of the opening of the West was the removal of the Native Americans who already occupied the land. Removal started during Andrew Jackson’s presidency with the Indian Removal Act of 1830, which authorized the president to force the removal of Midwestern tribes to reservations in Oklahoma and elsewhere. By the early 1860s, the U.S. government had systematized this “Indian territory” into small reservations and, in 1867 set aside two large tracts of land—one north of Nebraska and one south of Kansas—for tribal resettlement. The threat of force convinced many tribes to comply with resettlement. But some tribes, the Sioux in particular, fiercely resisted. In 1874, the U.S. Army sent Colonel George Armstrong Custer into South Dakota to fight the Sioux. At the Battle of Little Bighorn, in 1876, the Sioux crushed Custer and his men. After this defeat, the Army adopted a different tactic by launching a war of attrition, persistently harassing the Sioux and gradually weakening their will to resist. U.S. forces finally vanquished the Sioux in the Battle of Wounded Knee in 1890. Over the next decade, the Sioux relocated to reservations.

Not all Americans supported such aggressive removal tactics. Helen Hunt Jackson’s A Century of Dishonor, published in 1881, attempted to raise public awareness of the Native American plight. Some hoped to “save” the Native Americans through religion, or to “civilize” them by teaching them white ways. Other humanitarians suggested that the best approach would be to fully integrate the tribes into white society.

These latter concerns were expressed in the 1887 Dawes Severalty Act (or simply the Dawes Act), which called for the breakup of the reservation system and the treatment of Native Americans as individuals rather than as tribes. Congressman Henry Dawes believed that private land ownership would help Native Americans become “civilized” and assimilated. Under the act, formally communal land from the reservations was distributed to individuals in 160-acre allotments, and these individuals were guaranteed U.S. citizenship after twenty-five years. The surplus land that remained of the reservations after these allotments had been made was sold to white settlers and land speculators. In practice, much of the land parceled out to Native Americans wound up in white hands after poverty forced many Native Americans to sell their plots. As a result, many Native Americans were left homeless, destitute, and dependent on federal aid for survival. Though passed with good intent, the Dawes Act had disastrous effects: it disintegrated tribal communities and deprived Native Americans of millions of acres of land, clearing the way for American settlement in the process.

Another factor impairing the Native American way of life was the mass slaughter of buffalo. Many Plains tribes depended on buffalo for food, leather, and other material needs. But by the 1870s the buffalo population hovered near extinction, as white hunters killed 9 million buffalo between 1872 and 1875. American hunters often killed the animals solely for their hide, leaving the carcass to rot, while Army generals killed the buffalo in deliberate attempts to drive Native Americans off of desired lands.

To promote settlement of the West, Congress passed the Homestead Act in 1862, which offered 160 acres to anyone who would cultivate and improve the land. Much of this western land, however, was ill-suited to farming, so ranchers and railroad builders ended up owning most of it.

Another way Congress spurred settlement was by extending the railroad network into the West. In 1862, Congress passed the Pacific Railway Act, which chartered the Union Pacific Railroad company and authorized the building of the transcontinental railroad. While the Union Pacific Railroad Company built tracks westward from Iowa, the Central Pacific Railroad Company built tracks eastward from California. The two tracks converged on May 10, 1869 in Promontory, Utah. This historic moment marked the completion of the first transcontinental railroad, and by 1884, there were four such lines. Whereas just fifty years earlier it had taken pioneers many grueling months to cross the nation, Americans could now travel from coast to coast in a week’s time. Railroads attracted many new settlers to the newly accessible West by offering free transportation and long-term loans to travelers.

The expansion of the U.S. into the West reopened a controversy that had been temporarily settled by the 1821 Missouri Compromise: the balance of slave-holding versus free lands. Regional passions flared as the nation debated the extension of slavery into the new territories. In 1844, Congress repealed the 1836 gag rule, which had suppressed all debates on slavery, and disputed the status of the newly acquired territories. Texas entered the Union as a slave state in 1845 because the territory was already slave-holding when it sought admission. But the other lands ceded by Mexico—including California and New Mexico—were undecided, so Northern and Southern interests rallied to recruit these lands to their side.

In 1846, Democratic congressman David Wilmot attempted to preempt the debates that would erupt when the U.S. gained additional western lands by proposing the Wilmot Proviso, which stipulated that slavery be prohibited in any territory gained from Mexico. With strong support from the North, the proviso passed through the House of Representatives but stalled in the Senate, where it was repeatedly reintroduced without success. The issue sparked intense sectional debate. In the debates, four main arguments emerged:

Before the 1848 election, antislavery advocates united to form the Free-Soil Party and nominated Martin Van Buren for president. The Free-Soil Party consisted of antislavery Whigs, members of the abolitionist Liberty Party, and a faction of the Democratic Party (known as the Barnburners) that supported the Wilmot Proviso. Although the Free-Soil Party did not win any electoral votes, it did earn 10 percent of the national popular vote. Van Buren lost the election to Whig candidate Zachary Taylor.