Final answer:
Manifest Destiny was a key reason for westward expansion in the United States. It was the belief that America had a duty to expand westward and spread its ideals.
Step-by-step explanation:
The concept of Manifest Destiny was a key reason for westward expansion in the United States. Manifest Destiny was the belief that it was America's destiny and duty to expand westward and spread American ideals and institutions across the continent. This belief was rooted in the idea that Americans were superior and had the right to control the land and its people.