What were the effects of Manifest Destiny?
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.
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.