"inevitable and justified"—essentially that God had ordained this expansion to give the fertile land of America to these people.

The belief and justification of this idea led to many Americans deciding that it was their responsibility and right to lay claim to the Western frontier—and that everything to the Pacific Ocean was their right to take control of and use. This "personal responsibility" encouraged people to travel to the West much more quickly and frequently than they would have otherwise, spurring a drastic spike in its population and creating small towns and homesteads that dotted the land from Missouri to California.

By supporting this belief, it encouraged the American people to do whatever was necessary to take these lands—whether it was killing off wildlife or driving out remaining Native Americans. Instead of pausing because of these obstacles, the expansionism went forward unimpeded, leading to the takeover of the American continent.

Manifest destiny is a term that is used to describe an expansionist ideology that took shape in the 1830s and 1840s. The phrase itself has its origins in an essay by John O'Sullivan, a journalist who argued that the United States, by virtue of being a white, democratic nation, represented progress and "futurity." Therefore, he claimed, the United States was divinely chosen to spread throughout the North American continent. In practical terms, this was an ideology that made its way into mainstream American politics, particularly the Democratic Party. Its promoters vigorously advocated the annexation of Texas, newly independent from Mexico, in the 1830s. They also backed an aggressive American stance in the Oregon Territory, jointly claimed by the United States and Great Britain. James K. Polk, the president most identified with this ideology, pushed for, and got, a war with Mexico that resulted in the acquisition of almost all of the modern American Southwest, including California. So, in short, the ideology of manifest destiny influenced American expansion in the antebellum period by vigorously promoting it. This ideology was controversial even in its own time, however, and many Northern politicians began to believe that expansionism was aimed at promoting the interest of Southern slave states. Whether manifest destiny was the cause of expansion is a matter or debate, but at the very least it created the political and ideological atmosphere that promoted it.

The idea of manifest destiny, a term popularized by the newspaper journalist and editor John O'Sullivan in the mid-1840s, asserted that it was completely obvious (manifest, apparent, not arguable) that white citizens of the United States were fated (destined, meant) to control the entire North American continent. O'Sullivan used the term in newspaper columns to advocate in favor of the United States annexing the Texas and Oregon territories, which the country did do.

Manifest destiny influenced western migration by arguing forcefully that white Americans belonged in the West: God had set the land aside particularly for the use of members of this superior, Christian, and democratic society. Ideology is powerful, and manifest destiny became a powerful tool erasing doubts about whether the US should expand as well as doubts about the justice of displacing, and often killing, Native Americans to obtain their territory. After all, if God is on your side and what you are doing is obviously meant to be, it is easy to move forward with confidence.

Manifest Destiny was a concept that impacted our western expansion. The idea behind Manifest Destiny was that it was our duty or destiny to expand from the Atlantic Ocean to the Pacific Ocean.

This concept suggested that our way of doing things was superior to those of other groups. It implied it was G-d’s will for the American people to settle and to develop the land. We viewed the Native Americans as a group of people who were holding back progress. They needed to be relocated so we could expand. We viewed the Mexican control of the southwestern part of the United States as an impediment to progress. Thus, we went to war with Mexico so we could control this land and develop it. As we controlled more land, we spread our ways of living, which we believed were superior to other group’s ways of living.

The idea that we were the ones that should develop the land and spread our way of life helped to encourage people to move to the West so we could settle there and develop the region.

The idea of Manifest Destiny influenced the America’s westward migration because it made that migration more likely.  It made America more likely to expand territorially.

The idea of Manifest Destiny held that Americans were superior to most other people in a number of ways.  They were said to have a superior form of government, a superior culture, and a superior religion.  For these reasons, it was said, they were destined by God to expand their territory. 

This attitude led to the westward migration.  First, it helped lead to the expansion of US territory.  It helped lead to the Mexican-American War because it promoted the attitude that America deserved all of that land more than the Mexicans did.  America’s God-given destiny made it acceptable to take the land from Mexico.  The same attitude applied to the Indians as American settlers moved west.  The idea was that the Indians were so inferior that they did not deserve the use of the land.  They were to be moved and the land was to go to the Americans who deserved it and would use it well.

In these ways, the idea of Manifest Destiny helped to cause the western expansion of the United States.