John O'Sullivan coined the phrase Manifest Destiny in 1845, and used it to capture the overall feeling of the American people at the time, which was that it was their God-given duty to expand into the West, bringing the enlightenment of the American government with them. John O'Sullivan's article was written shortly after the annexation of Texas, and the phrase inspired further expansion.
Manifest Destiny took the form of aggressive expansion into the west, which was mainly occupied by Native Americans at the time. As a result, there were many wars that were justified using the Manifest Destiny rhetoric.
At the time, the American people had strong, Puritan religious beliefs, which led them to the conclusion that bringing the values of the U.S.A to what were regarded as primitive cultures (the Native Americans and Mexicans) was the work of God.
Although the phrase is often linked with western expansion on the American continent, it was also used as a driving force behind the involvement in a number of international wars.
The Spanish-American war is one such example, after which the Philippines, Guam, and Puerto Rico were conceded to America by the Spanish.
Manifest Destiny is also thought to have been directly linked to (and responsible for, to some extent) the overly imperialistic attitudes of U.S. citizens at the time.