“We need Greenland from the standpoint of national security,” President Donald Trump said on Jan. 5, “and Denmark is not going to be able to do it.” This was the icebreaker for a negotiation over Greenland’s reserves of “critical minerals,” especially rare earth elements. The outcome may lead to a gold rush as American companies extract next-generation resources from a new frontier in the freezing north. It may confirm the polarity of American-Chinese rivalry, with 21st-century mineral politics augmenting 20th-century energy politics. It may also backfire by breaking America’s Atlantic alliances with NATO and driving Europe’s faltering economies closer to Russia and China. Trump’s loose talk about buying Greenland from Denmark or even conquering it grabbed the headlines and panicked European politicians. The Danes and their European allies sent a token force of human tripwires to Greenland. Trump retaliated with tariffs on eight European states, including Britain, France, and Germany, starting at 10% on Feb. 1 and rising to 25% on June 1. Privately, European officials called Trump “crazy” and “mad” and called his unprovoked “attack” a “step too far.” French President Emmanuel Macron called the tariff threat “unacceptable.” British Prime Minister Keir Starmer called it “completely wrong.” Trump was unmoved. “We have to have it,” he told reporters on Jan. 20. “They have to have this done. They can’t protect us.” While in Davos, Switzerland, Trump seemingly ruled out taking Greenland by force. He...