Manifest Destiny was the philosophy or belief that God had ordained Americans, specifically white Americans, to spread across the entire continent of North America, bringing with them capitalism and democracy. The term was first coined by a magazine editor named John L. O'Sullivan in the July-August 1845 issue of The Democratic Review.

Territorial expansion was facilitated by several agreements with foreign governments that enabled the United States to acquire vast areas of land. For instance, the Louisiana Purchase in 1803 almost doubled the size of the United States, giving it 828,000 square miles of land west of the Mississippi River. The Oregon Treaty of 1846 gave the United States present-day Washington, Oregon, Idaho, and parts of Wyoming and Montana. The Treaty of Guadalupe Hidalgo in 1848, which ended the Mexican-American War, gave the United States over 525,000 square miles more, comprising present-day California, Arizona, New Mexico, Colorado, Texas, Nevada, and Utah. The acquisition of all this territory along with belief in Manifest Destiny had a profound effect on numerous groups of people.

One of the groups affected by Manifest Destiny was lower and middle class white American farmers, tradesmen, and adventurers, who recognized the opportunity of acquiring a better life by settling on cheap land in the west. These were the people who were best poised to take advantage of the doctrine.

Manifest Destiny also profoundly affected the lives of Native Americans—but in a negative way. As settlers surged westward, many of them thought nothing of brutally mistreating and displacing Native Americans to take over their land. This ignited bloody wars between the Native Americans and the settlers. Native Americans ultimately lost and were relegated to reservations, usually on the worst patches of land.

Abolitionists, slave owners, and slaves were also affected by Manifest Destiny. Controversies erupted over whether acquired states and territories should allow or prohibit slavery. These disputes were not resolved until after the Civil War.

The rich were affected by Manifest Destiny. People made or increased fortunes by building railroads and shipping companies to unite the far-flung settlers and also by mining resources such as gold and silver.

Manifest Destiny affected the entire United States. Expansionist politicians were affected by this notion, as they were able to receive votes from Westerners who sought cheap land in the West. Any politician who stood in the way of US Western expansion often did not fare well in the next election. For example, Abraham Lincoln's Congressional career was not deemed a success after he spoke out against the Mexican War.

Native Americans were also profoundly affected by Manifest Destiny, as they lost nearly all of their land to the expansionist Americans. American farmers brought their diseases and their views on proper agriculture with them, to the detriment of native society. American railroads adversely affected the buffalo, an animal of spiritual and cultural importance for the Native Americans. The Native Americans were also affected by the forts created by the US army in order to support western expansion. The forts would serve as both an area of controversy as well as a trading post for Native Americans.

The increased availability of farmland also affected potential landowners looking to go West. Many settlers viewed the Oregon Territory as having the best farmland in the United States. Many Easterners came to California looking to make it rich in the gold industry. Settlers of all types came West in order to sell goods to their fellow countrymen. Though Americans believed it their God-given right to come West, many did so in order to find easier profits than they could back East.

Born in the 1840s, Manifest Destiny is the idea that America was destined by God to colonize the whole of North America. This idea was important in the formation of American identity during this period and impacted a significant proportion of the population.

Christian missionaries, for example, were inspired by Manifest Destiny because it provided them with the opportunity to spread the Christian message. In their eyes, the Native Americans were uncivilized heathens who ought to be converted to Christianity, and they experienced widespread prejudice and discrimination, including the loss of their lands, as a result. 

Manifest Destiny also had an impact on writers of this period. For instance, the poet Walt Whitman (1819-1892) wrote poems which romanticized the idea of westward expansion. One example is "Pioneers! O Pioneers!" which emphasizes the bravery and dedication of those who headed westward and settled the land. For more examples, see the reference link provided.

Manifest Destiny affected the lives of many different groups within the U.S.

Lower and middle class farmers were affected by the large tracts of land opened by territorial acquisitions and conquests. As the U.S. expanded its reach west, new lands were opened up for settlement. Farmers, shopkeepers, trappers and many other groups began settling in these lands, starting new lives and expanding the U.S. economy.

Other countries were affected by Manifest Destiny as well. Great Britain, France, Mexico, Russia and a number of other nations all saw their borders change or their overseas colonies shrink and grow thanks to American expansionism. Sometimes it was mutually beneficial, such as in the case of France and the Louisiana Purchase. Sometimes it wasn’t, such as in the cases of Spain’s loss of Florida or Mexico’s loss of its northern provinces.

The group most affected however were Native Americans. As settlers moved west, natives were pushed off their tribal lands or forced into losing battles with the American military. Sometimes they were pushed on to reservations, like in the case of the Navajo or Sioux. In other cases they were totally wiped out, like the Modoc of northern California. Diseases, Christian missionaries and alcohol also played a role in the destruction of the native way of life.