Manifest Destiny: The belief prevalent in the 19th century that it was the destiny of the United States to expand its territory across the entire North American continent.
Louisiana Purchase: A land deal between the United States and France in 1803, doubling the size of the nation by acquiring a vast area of land west of the Mississippi River.
Mexican-American War: A conflict fought between Mexico and the United States from 1846 to 1848, resulting in significant territorial gains for the U.S., including present-day California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado and Wyoming.