Frederick Jackson Turner was an American historian that defended that the strength and the vitality of America’s identity lay in its land and vast frontier.
He said that the vast frontier was very important for an - then young - country like America. He also said that America should always seek new frontiers abroad.
This changed people’s idea and America identity, this led the US to call for new frontiers like in Cuba, Philippines, Puerto Rico, and Guam.
Because of this growing frontiers, America became a diverse country with different countries and tolerant social mix, America is a safe place for many races and religions, all because of its frontiers.