In September 1939, following the outbreak of World War II, the Australian government decided to raise an all volunteer force for service overseas during the war. This decision was based upon the provisions of the "Defence Act (1903)" which precluded sending members of the Militia outside of Australian territory to fight. Meanwhile, the Militia was tasked with home defence, and the 3rd Brigade was assigned a command reserve role within Southern Command. At this time, units of the Militia were called upon to undertake defensive duties and to provide training for men that were called up for national service following the recommencement of the compulsory training scheme in January 1940. Initially this was undertaken in small blocks of continuous training, however, following Japan's entry into the war following the bombing of Pearl Harbor and the invasion of Malaya in December 1941, the 3rd Brigade was mobilised for wartime service.