President Trump again threatened on Friday to forcibly annex Greenland, saying that he was “going to do something on Greenland, whether they like it or not.” In a White House event discussing his plans to have American companies exploit Venezuela’s vast oil reserves under the threat of a military blockade, Mr. Trump advanced an imperialist vision of American foreign policy, where the U.S. must dominate strategically important neighboring countries because of the perceived possibility that rival powers might do so first. “If we don’t do it, Russia or China will take over Greenland,” Mr. Trump said, falsely suggesting that Greenland,...