1)
America is a settler nation. The majority of the United States population has always been of foreign descent. Early on this was primarily people of European and African descent, and to this day European-Americans remain probably the richest and most politically dominant group. That said, there were already numerous communities of indigenous people living throughout the lands that became the territory of the United States. Those communities, if not necessarily the individuals who made them up, had to go as political entities for the US to be born. That required an ideological justification not just for colonialism as seen in places like India but building a new society in the place of indigenous societies, and Manifest Destiny served that function well. But once America’s existence was justified based on a racial and political ideology that held America had both the right and duty to serve as a model of democracy and freedom that entitled it to replace the inferior indigenous societies it was a quick step from there to expanding over the backs and bones of other inferior indigenous societies to the West.
2)
Manifest Destiny provided the ideological justification for American westward expansion. Moreover, Manifest Destiny grew out of a racial and political ideology going back to America’s foundation that in many ways required westward expansion. That said, Manifest Destiny is not sufficient as a full explanation of American westward expansion. Empires have expanded over the bodies of weaker neighbors since time immemorial. The territory of the present-day United States represents a huge wealth of natural resources that a non-racist, Chinese-style dynastic would have desired every bit as much as American politicians and businessmen. Thus, while Manifest Destiny justified and undergird America’s westward expansion that expansion almost certainly would have happened on some pretext without it.
References
Dudley, W. Manifest Destiny. Retrieved from http://www.historynet.com/manifest-destiny
Montoya, M. How Did Manifest Destiny Shape the American West? The Gilder Lehrman Institute of American History. Pod cast retrieved from https://www.gilderlehrman.org/multimedia#!15743
