Since colonial times, many Americans had believed their nation had a special role to fulfill. The Puritans, for example, wanted to build their new settlement as a “city upon a hill”, which was a biblical allegory for a moral and spiritual example for the whole world. For years, people thought the nation’s mission should be to serve as a model of freedom and democracy, and the Revolutionary War and the War of 1812 seemed to prove that. In the 1800s, though, that vision changed. A new sense of nationalism swept over America, and many believed that the United States’ mission was to spread freedom by occupying the entire continent. In 1819, John Quincy Adams expressed what many Americans were thinking when he said expansion to the Pacific was as inevitable “as that the Mississippi should flow to the sea.”

In 1845, New York newspaper editor John O’Sullivan put the idea of a national mission in more specific words. O’Sullivan declared it was America’s “Manifest Destiny to overspread and to possess the whole of the continent which Providence has given us.” O’Sullivan meant that the United States was clearly destined, or set apart for a special purpose, to extend its boundaries all the way to the Pacific. The only problem with that was that the land west of the Missouri River watershed – the land gained in the Louisiana Purchase – was still owned by other nations, such as Spain, then Mexico. As well, there were hundreds of thousands of others living on that land – Native Americans. It would take compromises, negotiations, and war to see the vision of Manifest Destiny through, and from the 1830s to the late 1840s, America would grow at some cost.

There were really a few reasons that Americans felt they had the right to expand: religious certainty, economics, and superiority. America saw a new religious awakening in the early nineteenth century, known as the Second Great Awakening. Much like the first awakening almost a century before, it brought with a new fervor for religion, especially Protestant Christianity. New sects developed, as well as some that challenged conventional faith, like the Mormons. Others saw that faith and patriotism, a fervor for one’s nation, were connected. Many settlers believed that God himself blessed the growth of the American nation. As well, Native Americans were considered heathens, or non-believing peoples needing salvation. By Christianizing the tribes, American missionaries believed they could save souls and they became among the first to cross the Mississippi River.

That desire to “save the heathens” also was rooted the pervasive belief in American cultural and racial superiority. Perhaps a hangover from the English thoughts on the idea, Native Americans had long been perceived as inferior, and efforts to "civilize" them had been widespread even since colonial times. Additionally, the Spanish and then Mexicans who ruled Texas and California were also seen as "backward” and inferior. Part of that had to do with their views on democracy, but a larger part had to do with the anti-Catholic attitudes of many Protestant Americans. It was seen as a duty to spread American values to those who were not viewed as “correct”.

However, the biggest motive of all was economics. The fur trade had been dominated by European trading companies since colonial times, and Americans wanted in on the action. Additionally, there was the lucrative attraction of having Pacific ports to trade with Asia, as well as the potential mineral wealth of Spanish/Mexican lands, and the vast lumber of the Northwest. There was also one more financial boon to expansion, really seen by the South. The desire of southerners to find more lands suitable for cotton cultivation would eventually spread slavery to these regions. After expansion occurred, this would lead to conflict as north of the Mason-Dixon line, many citizens were deeply concerned about adding more slave states. Manifest Destiny touched on issues of religion, money, race, patriotism, and morality. These clashed in the 1840s as a truly great drama of regional conflict began to unfold.