Summary
Highlights
Both the U.S. and Great Britain claimed the Oregon Territory. In 1846, the two countries agreed to settle the dispute, with the U.S. receiving all territory south of the 49th degree latitude line.
Manifest Destiny refers to the belief that it was America's destiny to extend its territory from the Atlantic to the Pacific Ocean. Motivations for this expansion included the demand for new farmland due to population growth, religious refuge (like the Mormons settling in Utah), and the California Gold Rush of 1848.
In 1803, President Thomas Jefferson acquired the Louisiana Territory from France's Emperor Napoleon for $15 million. This purchase doubled the size of the United States and gave it full control of the Mississippi River, including the port of New Orleans. Meriwether Lewis and William Clark, with Sacagawea as their guide, explored and surveyed this new territory.
Following raids by Native Americans and the refuge of runaway slaves in Spanish-controlled Florida, General Andrew Jackson invaded Florida in 1818. In 1819, Secretary of State John Quincy Adams negotiated the Adams-Onís Treaty, in which Spain ceded Florida to the United States.
American families began settling in Mexican Texas in the 1820s. Mexico closed Texas to American emigration and banned slavery in 1830, leading to a war for Texan independence (1835-1836). Despite the defeat at the Alamo, Texas forces led by Sam Houston won at the Battle of San Jacinto. Texas was an independent republic until its annexation into the U.S. in 1845 by President James Polk.
In 1846, reported firing by Mexican soldiers on American forces under General Zachary Taylor led President Polk to declare war. The Mexican-American War (1846-1848) concluded with the Treaty of Guadalupe Hidalgo. This treaty resulted in the U.S. acquiring present-day California, Nevada, Colorado, Utah, Arizona, and New Mexico.
By 1853, the U.S. had acquired all the territory that makes up the present-day contiguous United States. This expansion led to controversies, especially with Native American tribes, and intensified the debate over slavery, which would lead to boiling point in the 1850s.