The declaration of World War II in September 1939 led to Australia's involvement and to the systematic expansion of defence sites which had begun in the mid-1930s. The construction of prefabricated and pre-cut structures is consistent with the rapid increase in military facilities around Australia which followed the Japanese attack on Pearl Harbour and America's entry into the war. Australian concerns as to the safety of Australian territory were borne out when Darwin was bombed by the Japanese in February 1942.