Explanation-
Frederick Jackson Turner's frontier thesis argued that the American character and institutions had been shaped by the existence of a western frontier. This thesis influenced the expansionist mood in the US by providing a rationale for the acquisition of new territories. Turner argued that the frontier was a place where Americans could escape the constraints of civilization and reinvent themselves. He also argued that the frontier promoted democracy and equality.
These ideas resonated with many Americans at the time, who were eager to expand the nation's borders. The frontier thesis helped to justify the annexation of Texas, the Mexican-American War, and the acquisition of Alaska, Hawaii, and the Philippines.