Political map of the Arctic & the Far North 6 April 1917 (Great War and the Arctic): In 1914 the Great War (World War I) broke out in Europe. While the United States kept out of the War for the first few years, it expanded in the Caribbean by purchasing the Danish West Indies (Treaty of the Danish West Indies), in return recognizing Danish sovereignty over all Greenland. Only days after completing this deal, the US joined the Allies, responding in part to German attempts to starve Britain into surrender through unrestricted submarine warfare (U-boat Campaign (World War I)).