During the 19th Century, more than 1.6 million square kilometers (a million square miles) of land west of the Mississippi River was acquired by the United States federal government. This led to a widespread migration west, referred to as Westward Expansion.

A variety of factors contributed to Westward Expansion, including population growth and economic opportunities on what was presented to be available land.

Manifest Destiny was the belief that it was settlers’ God-given duty and right to settle the North American continent. The notion of Manifest Destiny contributed to why European settlers felt they had a right to claim land, both inhabited and uninhabited, in western North America. They believed it was the white man’s destiny to prosper and spread Christianity by claiming and controlling land.

Manifest Destiny was used to validate the Indian Removal Acts, which occurred in the 1830s. Such legislation forced the removal of Native Americans and helped clear the way for non-native settlers to claim land in the west. When the settlers reached land populated or previously promised to Native Americans, they had no qualms claiming it for their own benefit.

It was not just spiritual prosperity that inspired settlers—outright moneymaking opportunities also motivated Westward Expansion.

Throughout most of the 19th century, there were two main ways to make money west of the Mississippi River: through gold and silver prospecting, and through developing land for agriculture, industry, or urban growth. These two activities often supported each other. In California, for instance, the actuality of “striking it rich” was quite short-lived, although immigrants continued to populate the new state and contribute to its agricultural and economic growth well after gold fields were discovered there in 1848.

The idea of “free land” was fairly short-lived as well. By 1890, the U.S. Census reported that there were so many permanent settlements west of the Mississippi that a western “frontier” no longer existed in the United States.

This declaration inspired a young historian, Frederick Jackson Turner, to write his famous “Frontier Thesis.” Turner claimed the “close of the frontier” was symbolic. He asserted that Westward Expansion was the most defining characteristic of American identity to date. With the close of the frontier, he thought, America was that much more “American”—liberated from European customs and attitudes surrounding social class, intellectual culture, and violence.

Many historians criticize the Frontier Thesis, and many reject the idea of an American “frontier” (which Turner described as “the meeting point between savagery and civilization”) entirely. These historians recognize that the “free land” that defined Westward Expansion came at a severe cost to Native American and Spanish-speaking populations, as well as more recent immigrants from Asia (who migrated east, across the Pacific). The Frontier Thesis ignores the development and evolution of these identities almost entirely.

To delve deeper into this complex period of American history, check out our curated resource collection page on Westward Expansion at the National Geographic Resource Library.