Manifest Destiny, the mid-nineteenth century expansionist movement, was instrumental in shaping the United States and in enabling it to reach its geographical potential and diversity. Had Manifest Destiny not occurred, the US would probably not extend further West than the Mississippi River. And the US would probably not have become the great world power it is today.

Although the term Manifest Destiny was not coined until 1845, the idea existed in the minds of Americans and their leaders long before. The Louisiana Purchase of 1803 greatly increased the country's size. American settlers helped Texas win its independence from Mexico in 1836.

Mexico, in fact, became a victim of American aggression fueled by Manifest Destiny. Mexico lost Texas in 1836, and it lost nearly half of its territory after being defeated in the Mexican-American War (1846–1848).

Another effect of Manifest Destiny was that it exacerbated tensions between the North and the South during the years preceding the Civil War (1861–1865). Both the North and the South hoped to create new states that would buttress their position in the Senate. The tension helped create the Republican party and facilitate Abraham Lincoln's rise to the presidency.

Groups that were victimized by Manifest Destiny included native Americans and Hispanics. The rights of indigenous and Spanish-speaking peoples were almost entirely ignored as white settlers pushed West.

Finally, Manifest Destiny was a safety valve for discontented white Americans who decided to seek their fortunes in the West.

The main effect of Manifest Destiny was American expansion. The bulk of this expansion came through the annexation of Texas and the acquisition of lands from the Mexican War. In fact, the phrase "Manifest Destiny" was coined to describe the belief that the United States should take lands from Mexico, which was seen as incapable of administering them. This attitude led to war with Mexico, a conflict which was vigorously supported by expansionists. So a major effect of this ideology was the expropriation of lands from Mexico. Though an antebellum concept, Manifest Destiny also entailed the taking of lands from Native Americans that occurred before, during, and after the war. Finally, another indirect effect of Manifest Destiny was that it worsened the sectional crisis that brought about the Civil War. This is because Southerners demanded that slavery be allowed in Western territories, and many Northerners opposed it. The intense disagreement was only temporarily resolved by the Compromise of 1850. So Manifest Destiny and westward expansion in general contributed to the outbreak of the Civil War.

Manifest Destiny forms the rationale behind American imperialism. The concept suggested that it was the responsibility of the European settlers to colonize and bring civilization to their new environment. The belief also fueled the Westward Expansion, which led to mass relocations of the Native Americans. These relocations were followed by annexation of Texas under James K. Polk, the President of the United States at the time.

The United States was also set for further expansion when it forced the extension of the boundary with Canada and later with Mexico. Some of the territories were acquired through conquest and others through purchase. The insatiable demands for more territorial expansion saw the US making moves on Cuba after helping rebels in their fight against Spain. Conflict with Spain (as in the Spanish-American War) saw the US acquisition of the Philippines, Guam, and Puerto Rico.

For the United States, the effects of Manifest Destiny were pretty positive.  The main impact was that the United States got much larger and much wealthier.  Manifest Destiny was the idea that God had destined the US to spread out across the continent of North America.  It was not specified how much land the US was to take, but people believed that God wanted the country to expand.  God had destined the US for this because it had the best religion, the best people (racially) and the best culture.  This led to things like the war with Mexico in the 1840s and all of the military actions that pushed Indians off their lands and on to reservations.  These actions greatly increased the size of the US and also made it richer by allowing it to take things like the gold of California and the silver of Nevada.

For others, Manifest Destiny had more negative effects.  For the Native Americans and for Mexico, it meant losing their land.  The Native Americans who were not killed outright in the wars were forced on to reservations where they lost their ability to live freely and to pursue their traditional way of life.  As a country, Mexico was humiliated and weakened by being defeated in a war and by having so much territory taken away.  Mexicans who lived in the areas taken by the US typically found themselves losing wealth and status as they came to be seen as foreigners in their new land.

In these ways, Manifest Destiny affected different groups in different ways.