The belief held by many Americans in the mid-19th century that it was their destiny and duty to expand westward across North America, spreading democracy and American values.
Related terms
Westward Expansion: The movement of Americans towards the western frontier, acquiring new territories such as Oregon, Texas, and California.
Louisiana Purchase: The acquisition of a large territory from France in 1803, which doubled the size of the United States overnight.
Mexican-American War: A conflict between the United States and Mexico from 1846 to 1848, resulting in significant territorial gains for the U.S., including California and New Mexico.