What, according to John O'Sullivan, is America's mission?

In 1839, John L. O'Sullivan, an American journalist wrote a article justifying the territorial expansion of the United States. According to O'Sullivan, the United States was a people chosen by God and destined to expand throughout North America. According to his article, the expansion of America was not an option, but a destination the Americans could neither avoid, because they would be rejecting the will of God.