How did Manifest Destiny affect Americans' ideology during the mid-1800s?
To the extent that the United States of America comprised a specific, carefully delineated geographic region, beyond which was land not formally part of the United States, then the articulation of a Manifest Destiny could be seen as having constituted the first, and most cohesive, foreign policy in the nation's history. As the eNotes essay, the link to which is attached below, points out, the inclination to expand westward was given its formal statement of purpose in an 1845 article by newspaper editor John L. O'Sullivan. The United States of America, he argued, had "the right of our manifest destiny to spread and to possess the whole of the continent which Providence has given us for the development of the great experiment of liberty and federative development of self government entrusted to us. It is right such as that of the tree to the space of air and the earth suitable for the full expansion of its principole and destiny of growth."
Whereas, as noted, the United States was already possessed of a sense of westward expansion, the notion of a "manifest destiny" was institutionalized as U.S. foreign policy with the publication of O'Sullivan's editorial. It thereafter became U.S. policy to spread as far as the landmass allowed, with any and all obstacles justified in their destruction. In that sense, Manifest Destiny became an American ideology, and one that propelled the nation toward ever-greater expansion.