The expansion of the United States into the territory west of the Mississippi River began with the Louisiana Purchase in 1803. President Thomas Jefferson nearly doubled the size of the nation by negotiating a price of $15 million to purchase 828,800 square miles from France, including all or part of 14 current states. In 1804, Jefferson sent an expedition led by Meriwether Lewis and William Clark to explore the area. The three-year expedition produced new understanding of the geography and resources of the western part of the continent.

Prior to that time, European immigrants did not stretch far beyond the eastern seaboard. The first British settlers in the New World stayed near the Atlantic Ocean to be close to their lifeline for needed supplies from England. By the 1630's, however, Massachusetts Bay colonists were pushing into the Connecticut River Valley. Resistance from the French and the Indians also slowed westward movement; but, by the 1750's, colonists were living in most of New England. It took American colonists a century and a half to expand as far west as the Appalachian Mountains, a few hundred miles from the Atlantic coast. It took another fifty years to push the frontier to the Mississippi River.

Despite some continuous push westward, it was not until the conclusion of the War of 1812 that the westward movement became a significant outpouring of people across the continent. By 1830, the Old Northwest and Old Southwest, which were scarcely populated before the war, began to be settled and before long, the states of Illinois, Indiana, Missouri, Alabama, and Mississippi were admitted into the Union.

In the 1830's and 1840's, “Manifest Destiny,” the idea that the United States was destined to expand across the entire continent, was used to promote further territorial expansion. The concept of American expansion was much older, but John L. O'Sullivan coined the term "Manifest Destiny" in the July/August 1845 issue of the United States Magazine and Democratic Review in an article titled Annexation. It was primarily used by Democrats to support the expansion plans of the Polk Administration, but the idea of expansion faced opposition from Whigs like Henry Clay, Daniel Webster, and Abraham Lincoln, who wanted to deepen the economy rather than broaden its expanse.

Americans began to buy into the inevitability of settling unexplored western frontiers, first moving into places such as Michigan, Arkansas, Wisconsin, and Ohio. In the 1840's and 1850's the nation expanded quickly and in the span of just five years, the United States increased its size by a third.

It annexed Texas; negotiated with Britain for half of the Oregon country; and acquired more as a result of a war with Mexico as noted in this timeline:

• In 1845, the United States annexed Texas.

• In 1846 the Oregon Treaty ended British claims to Oregon Territory.

• In 1848, following the Mexican-American War, Mexico ceded much of the West and Southwest to the United States. This included what would become the states of California, Nevada, Utah, parts of Arizona, Colorado, New Mexico, and Wyoming.

• In 1853 the United States bought an additional tract of land from Mexico.

People then began to move in numbers into Texas, California, and Oregon. The California Gold Rush, the construction of railroads, the Mormons' long pilgrimage to Utah, and the blazing of the Santa Fe and Oregon Trails all contributed to the expansion of the "wild west."