Manifest Destiny was an idea that implied that the United states had the divine right (i.e. the God-given right) to expand to the Pacific Ocean. The central idea of this position was that the US had a divine right to spread across the continent of North America and into the Pacific Ocean. A newspaper man named John L. O'Sullivan first used the term in 1845. President James K. Polk ran on a platform of what would be called Manifest Destiny. A belief in Manifest Destiny led to the U.S. fighting the Mexican-American War. It involved convincing Americans that the reason it was okay to fight, kill take the land from Mexicans or other non-Americans, was because God (or possibly the spirit of "Columbia") wanted them to cause this suffering. This belief continued to play a key role in the United States settlement of western lands, especially in the rush to California and in the U.S. taking over Oregon.
This was being rude to the natives by taking there land and that is why some people hesited or were apposed to the U.S. Expasion