Answer:
An argument used by John L. O'Sullivan to explain the idea of Manifest Destiny was that the United States was unique in the world as a democracy of people from many different nations.
John L. O'Sullivan (1813-1895) was an American Journalist that coined the term "Manifested Destiny" in 1845. He was writing a column when he applied the term when he was referring to the annexation of the Oregon county and the Texas territory to the US. He published his essay called "Annexation" in the newspaper called "Democratic Review." An argument used by John L. O'Sullivan to explain the idea of Manifest Destiny was that the United States was unique in the world as a democracy of people from many different nations.