Melting polar ice is thrusting the Arctic Circle into the geopolitical limelight. The frigid region is estimated to contain nearly a third of the world’s undiscovered natural gas and 15 percent of its undiscovered oil, and the Arctic countries—Russia, Canada, Denmark, Norway, and, thanks to Alaska, the United States—are eying those resources as they jockey for position. But thawing ice will also expose new shipping lanes, and nations like the United States will have new responsibilities to keep those routes safe for commerce.