“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.