Which phrase summarizes the motivation behind American expansionism in the 19th century?

The phrase "Manifest Destiny" succinctly captures the motivation behind American expansionism in the 19th century. This concept embodied the belief that the United States was divinely ordained to expand its territory across North America, stretching from the Atlantic to the Pacific Ocean. Proponents of Manifest Destiny argued that this expansion was not just beneficial for the nation but was also a moral imperative, promoting democracy and civilization.

Supporters saw it as a way to spread American values and institutions, framing territorial expansion as a noble pursuit. The philosophy influenced numerous policies and actions, including the annexation of Texas, the acquisition of Oregon, and the westward push that led to conflicts such as the Mexican-American War.

The other phrases do not encapsulate this broader ideology as effectively. "Strength through isolation" reflects a different viewpoint that emphasizes staying out of foreign affairs, while "The frontier is closed" describes a condition that arose later in the 19th century, signaling the end of westward expansion. "Destiny to rule" is somewhat related, but lacks the specific historical context and widespread acceptance that characterized Manifest Destiny as a driving force during that era. Thus, "Manifest Destiny" serves as the definitive phrase representing the motivations behind American expansionism at that time.