On Friday, President Donald Trump cautioned that the United States might levy tariffs on nations that refuse to back his effort to bring Greenland under American control, heightening tensions with European allies over the strategically significant Arctic territory.Trump has a history of using tariffs and economic pressure as tools of foreign policy. His administration has imposed penalties on countries purchasing Russian oil and recently introduced broad sanctions targeting nations engaged in trade with Iran. He has now extended this approach by warning of tariffs against countries that oppose his plan to assume control of Greenland.