The correct answer is manifest destiny.
Manifest destiny is the belief that it was America's god given right to expand from "sea to shining sea." In other words, it was America's destiny to expand their control across the entire North American continent from the Atlantic to the Pacific Ocean. This belief resulted in the US buying land from other countries, fighting in wars, and taking land from Native Americans in order to achieve this goal. This was present throughout American society during the early to middle of the 19th century.