- Mexican-American War: The US won the Mexican-American War (1846-1848), which resulted in the US acquiring a vast amount of territory from Mexico, including the present-day states of California, Nevada, Utah, Arizona, and New Mexico, as well as parts of Colorado and Wyoming.
- Treaty of Guadalupe Hidalgo: The Treaty of Guadalupe Hidalgo, which ended the Mexican-American War, officially ceded these territories to the United States.
- Oregon Treaty: In addition, the US also acquired the Oregon Territory in 1846 through the Oregon Treaty with Great Britain, which established the US-Canada border at the 49th parallel.