The Second World War brought immense change to the archipelago. The fall of France to Nazi Germany in 1940 allowed Britain to gain a level of greater authority on the islands. The Australian military stationed a 2,000-strong force on Malakula in a bid to protect Australia from a possible Japanese invasion. Following the Japanese attack on Pearl Harbour on 7 December 1941 the United States joined the war on the Allied side; Japan soon advanced rapidly throughout Melanesia and was in possession of much of what is now Papua New Guinea and the Solomon Islands by April 1942, leaving the New Hebrides on the frontline of any further advance. In order to forestall this, from May 1942 US troops were stationed on the islands, where they built airstrips, roads, military bases on Efate and Espiritu Santo, and an array of other supporting infrastructure.