The fall of Denmark in April 1940 left the Danish colony of Greenland an unoccupied territory of an occupied nation, under the possibility of seizure by the United Kingdom, United States or Canada. To forestall this, the United States acted to guarantee Greenland's position. With the entrance of the United States into the war in December 1941, Greenland became a combatant.
From 1941 until 1945, the United States established numerous and extensive facilities for air and sea traffic in Greenland, as well as radio beacons, radio stations, weather stations, ports, depots, artillery posts, and search-and-rescue stations. The United States Coast Guard also provided a considerable portion of civilian resupply along both coasts.
Economically, Greenland traded successfully with the United States, Canada and Portugal, which, supplemented by the cryolite exports, caused a reanimation and permanent realignment of the island's economy.