Curtin led Labor to the 1937 election against Joseph Lyons' United Australia Party which resulted in a comfortable victory to the UAP.

The Labor Party declined to enter a war cabinet led by Menzies, though Curtin offered co-operation – though not to the extent of supporting conscription for overseas service.

[2] Curtin took a seat on the newly created Advisory War Council in October 1940 and agreed to a plan by Menzies to travel to Washington and London.

[2] In January 1941, Menzies flew to Britain to discuss the weakness of Singapore's defences and sit with Winston Churchill's British War Cabinet.

In Menzies's absence, Curtin co-operated with Deputy Prime Minister Arthur Fadden in preparing Australia for the expected Pacific War.

Returning to Australia, with the threat of Japan imminent and with the Australian army suffering badly in the Greek and Crete campaigns, Menzies re-approached the Labor Party to form a War Cabinet.

The UAP-Coalition held office for another month with Arthur Fadden of the Country Party as its leader, before the independents switched allegiance and John Curtin was sworn in as Prime Minister on 7 October 1941.

Curtin went on to lead federal Labor to its greatest win with two-thirds of seats in the lower house and over 58 percent of the two-party preferred vote at the 1943 election.

While demanding reinforcements from Churchill, on 27 December 1941 Curtin published an historic announcement:[8] "The Australian Government...regards the Pacific struggle as primarily one in which the United States and Australia must have the fullest say in the direction of the democracies' fighting plan.

US President Franklin Roosevelt ordered his commander in the Philippines, General Douglas MacArthur, to formulate a Pacific defence plan with Australia in March 1942.

Curtin hoped this might influence the British to despatch men and equipment to the Pacific, and the appointment reaffirmed the important role of the Crown to the Australian nation at that time.

He returned to Australia and campaigned for an unsuccessful 1944 referendum to extend federal government power over employment, monopolies, Aboriginal people, health and railway gauges.

[8] In April 1945, Curtin despatched an Australian delegation which included attorney-general and minister for external affairs H V Evatt to discuss formation of the United Nations.

[13] Following his 1945 election as leader of the Australian Labor Party, Chifley, a former railway engine driver, became Australia's 16th Prime Minister on 13 July 1945.