The notion of Manifest Destiny helps to illuminate a fundamental difference in both the experience of American History as well as the retelling of it. The O'Sullivan phrase was meant to express American belief in superiority and provided the rationale behind United States Expansionism. For a nation that had been victorious against England, twice, and for one that had been developing new notions of economic progress, to expand was seen as something that was both needed and mandated by the idea that America had been "chosen" to lead. The confidence in United States Exceptionalism is underscored by the belief that America is unique to the world scene, and thus it has a unique destiny to lead over all. This helps to illuminate a "consensus" view of telling American History, which stresses the belief in American progress and success as essential to the narrative of American History. The opposite point of view of this would involve seeing Manifest Destiny as something that usurped land and power from other nations and people, that defined American progress at the cost of others, and a vision of economic progress controlling all aspects of American Identity. Within this vision of Manifest Destiny would involve a telling of American History that is steeped in a "conflict" view, which details that the progression of American History is one where conflict and crisis helps to define American notions of power and superiority. Manifest Destiny is a period in American History where individuals are poised, to some extent, in making choices between both how the history is viewed and how it is told.
Manifest Destiny has been a major force in US history. It was very important in causing the expansion of the US in the 1800s.
Let's define it as the idea that A) America and American civilization should take control of as much land as possible and B) that this should be done because Americans had a culture and government that were superior to those of the people around them.
The major impacts of the concept were the expansion across America (the "winning of the West") and the Mexican-American War.
Does it define us as a people? It certainly defined us during those years. We used it as a justification for taking land from the less-developed people -- the Indians and the Mexicans and then overseas places such as the Philippines.
The idea that we should spread our ways (if not our actual control) to other people and other parts of the world is part of what has caused us to do things like invade Iraq in modern times.