The term Manifest Destiny as it relates to expansionism in the United States related to a widely held belief that that it was the destiny of the United States to expand westward across the American continent in order to spread democracy and liberty, and to provide additional resources and land for the good of the young American republic. Manifest Destiny eventually became synonymous with the goal of westward territorial expansion.
In his book "The American Story" Robert Divine makes reference to the idea that many Americans believed in the idea of Manifest Destiny, and that it was God's will that America expand westward. This notion of God's will or Divine Providence played an important role in justifying the actions necessary to expand westward. These actions included annexation of the land belonging to Native Americans and resulted in actions in direct violation of international treaties and agreements.
There were costs associated with this expansion. One was the outbreak of the Mexican American War. The Mexican-American War resulted from conflicts between Mexico and Texas. After gaining independence from Mexico in 1836, Texas was annexed by the United States in 1845. At the time, the borders between Texas and Mexico were in dispute which ultimately led to war as the United States claimed ownership of more land than the government of Mexico was willing to cede. It is interesting to note that the victory over Mexico expanded U.S. territorial holding significantly, which was proof to many Americans that territorial expansion was God's will which in turn justified decisions to break treaties with Native Americans in the quest to expand from the Atlantic to the Pacific Ocean (Divine et al).
Divine, Robert, T.H. Breen, George Frederickson, and R. Hal Williams. The American Story, Single Volume Edition . 2nd ed.: Longman, 2005.
...