Mexican-American War: The Mexican-American War was a conflict between Mexico and the United States from 1846 to 1848, resulting in Mexico ceding vast territories to the U.S., including present-day California, Nevada, Utah, Arizona, New Mexico, Colorado, Wyoming, and parts of Texas.
Louisiana Purchase: The Louisiana Purchase was a land acquisition made by President Thomas Jefferson in 1803 when he bought approximately 828,000 square miles of territory from France for $15 million. This purchase doubled the size of the United States.
Manifest Destiny: Manifest Destiny was a belief prevalent in the 19th century that it was America's destiny (fate) and right to expand its territorial boundaries from coast to coast. It influenced westward expansion and acquisition of new territories.