NUUK, Greenland — President Donald Trump wants to buy Greenland again. Since returning to the White House, he has revived talk of purchasing or annexing the world’s largest island from Denmark and refused to rule out using force to “get it one way or the other.” He argues Greenland is vital to U.S. security. Greenland may feel like an icy curiosity to most Americans, but who controls it could reshape Arctic shipping, missile defense, and power in the north. For all the criticism Trump’s plan gets, the island is emerging as the next frontier in the contest with China and Russia. The Trump administration is doing more than talking. In May, U.S. officials began exploring a Compact of Free Association with Greenland, a sweeping deal that could grant Washington expanded military access in exchange for economic...