Ranked colonel at the outbreak of the Second World War in 1939, Sturdee was raised to lieutenant general in 1940 and became Chief of the General Staff. He proceeded to conduct a doomed defence of the islands to the north of Australia against the advancing Japanese forces. In 1942, he successfully advised the government to divert the Second Australian Imperial Force troops returning from the Middle East to Australia. He then became head of the Australian Military Mission to Washington, D.C., where he represented Australia before the Combined Chiefs of Staff. As commander of the First Army in New Guinea in 1944–45, Sturdee directed the fighting at Aitape, and on New Britain and Bougainville. He was charged with destroying the enemy when opportunity presented itself, but had to do so with limited resources, and without committing his troops to battles that were beyond their strength.