Who first popularized the phrase "Manifest Destiny"?

John L. O'Sullivan (1813 – 1895) is generally credited as being someone who first popularized the phrase "Manifest Destiny". O'Sullivan, as a newspaper editor, coined the term "Manifest Destiny" in 1845 to describe the essence linked to the country's mindset. It was O'Sullivan believed that Providence (God) had given the United States a mission to spread republican democracy ("the great experiment of liberty"). US President James K. Polk (1845-1849) was then the country's leader, and he became greatly associated with the belief of "Manifest Destiny".

In simple terms, "Manifest Destiny" was the idea that Americans were destined, by God, to govern the North American continent. This idea, with all the accompanying transformations of landscape, culture, and implied religious beliefs, had deep roots in the American culture. It was also a justification, in that many Americans wanted territory and needed an excuse or justification for a push into territory that the country did not control.