The westward expansion of the United States was not a movement into territories previously unoccupied, but rather a series of movements into territories controlled either by Native Americans or by competing colonial powers.
Philosophically, in the 1850s there arose the notion of "Manifest Destiny", that the new United States was a culturally unique nation state, fated by its dynamism and moral rectitude to control land from the Atlantic to Pacific Oceans and Rio Grande north to the Canadian border; the northern expansionist aims were encapsulated in the memorable phrase "fifty-four forty or fight", an expression emphasizing the latitude demarcating the northwestern territorial ambitions of the United States.
The westward expansion put the United States in conflict with the plains Indian tribes, who required large territories for hunting. The northward expansion of the United States led to conflict with the British colonies of Canada. In the southwest, the United States ran into conflicts leading to the Mexican–American War.
Many current tensions between the descendants of European settlers and Hispanic and Native American populations within the United States derived from this period of conquest.