Final answer:
Manifest Destiny was the belief that the United States was destined to expand its territory from the Atlantic Ocean to the Pacific Ocean. It was tied to a sense of divine right and mission, and the idea that spreading American democracy was spreading freedom itself.
Step-by-step explanation:
The belief that the United States was to spread its borders from the Atlantic Ocean to the Pacific Ocean is known as Manifest Destiny. This term encapsulates several ideas: that the expansion of the United States across North America was a divine right, that it was the destiny of the country to spread democratic institutions from coast to coast, resulting in the Americanization of all inhabitants, and that this process was equivalent to spreading freedom. The concept became a driving force behind the westward expansion of the 1840s, leading to the annexation of territories like Texas and Oregon, and was further illustrated by America's actions towards overseas expansion in the later 19th century.