Greenland is indisputably the front line of American homeland defense. For decades, Washington relied on a passive 1951 security framework that left the vast, icy periphery vulnerable to great-power maneuvering. That changed on Tuesday, when a historic trilateral defense pact in New York executed a vital strategic pivot. By expanding Pituffik Space Base, reclaiming the former U.S. World War II airfield at Narsarsuaq, and securing a new military footprint at the Danish outpost of Mestersvig, our country moved past passive neglect. Washington now commands the shortest polar transit routes separating Eurasia from North America.History warns against complacency in this frozen...