The main idea behind "Manifest Destiny" in the United States was the belief that Americans were destined by God to expand their territory across North America, from the Atlantic to the Pacific Ocean. This ideology emerged in the 19th century and was used to justify westward expansion and the acquisition of new lands, often at the expense of Native American tribes and Mexican territories. Proponents argued that this expansion was not only a right but also a mission to spread democracy and civilization. This belief significantly shaped U.S. policies and actions, leading to conflicts such as the Mexican-American War and the displacement of indigenous peoples.