Answer: A moral justification of territorial expansion called manifest destiny, that fueled Americans to extend their territories from the Atlantic to the Pacific.
Step-by-step explanation:
The Great Plains and the Far West were already occupied by Indians and Hispanics, who had lived in the region for centuries and had established their own distinctive customs and ways of life. Historians estimate that over 325,000 Indians inhabited the Southwest, the Great Plains, California, and the Pacific Northwest in 1840 when the great migration of white settlers began to pour into the region. Many tribes resided on the Great Plains, a vast grassland stretching from the Mississippi River west to the Rocky Mountains and from Canada south to Mexico. This region had been virtually devoid of a human presence until the Spaniards introduced the horse and the gun in the late sixteenth century.