What ideology promoted the belief in the necessity of expanding the United States westward?

The correct choice is Manifest Destiny, which encapsulated the belief that the expansion of the United States across the American continent was both justified and inevitable. This ideology was rooted in the notion that the U.S. had a divine obligation to spread its values and democracy, which was seen as a beneficial force for both the nation and its inhabitants. Advocates believed that westward expansion would bring progress, civilization, and economic opportunities, not only to the U.S. but also to the indigenous peoples, whom they often viewed as obstacles to this vision.

Manifest Destiny was particularly influential in the 19th century and played a crucial role in events such as the annexation of Texas, the Oregon Trail migration, and the Mexican-American War. These occurrences were often framed as fulfilling America's destiny to expand, leading to significant territorial gains and shaping the national identity.

Other ideologies, like sectionalism, primarily focused on the divisions between the North and South, often regarding economic and political interests rather than the idea of expansion. Populism emerged later, addressing the concerns of the common people against the elite, while isolationism advocated for a policy of remaining apart from the affairs of other countries, which is contrary to the active pursuit of expansion that characterized Manifest Destiny.