Following the outbreak of World War I on August 4, 1914, Australia enthusiastically pledged its support alongside the British Empire and almost immediately began preparations to send forces overseas to participate in the conflict. The first significant Australian action of the war was the Australian Naval and Military Expeditionary Force's landing at Rabaul. It took possession of German New Guinea at Toma and of the neighbouring islands of the Bismarck Archipelago by October 1914. On April 25, 1915 members of the Australian Imperial Force (AIF) landed in Gallipoli with troops from New Zealand, Britain, and France. This began a campaign that ended with 26,000 Australian casualties and the evacuation of troops on December 19-20, 1915. Following Gallipoli, Australian forces fought campaigns on the Western Front and in the Middle East. For Australia, WWI remains the most costly conflict in terms of deaths and casualties. From a population of fewer than five million, 416,809 men enlisted, of which over 60,000 were killed and 156,000 wounded, gassed, or taken prisoner. For many Australians the nation's involvement in World War I and the Gallipoli campaign was seen as a symbol of its emergence as an international actor, while many of the notions of the Australian character and nationhood that exist today have their origins in the war.