SEATTLE—The 40-year-old Coast Guard icebreaker Polar Star returned to the Arctic Ocean this summer after seven years in semiretirement, charging into a thinning polar ice sheet that U.S. defense officials predict will give way to new commercial waterways and a resource-rich frontier by midcentury.

Navy officials say the Arctic will give the U.S. its first new ocean to police since the annexation of the Pacific Northwest in 1846. As the ice surrounding the North Pole retreats, officials say, commercial shippers will be able...