Question: What event caused the Liberal Party of Australia's longest-serving leader to become Prime Minister?

Knowledge 1: Liberal Party of Australia
The contemporary Liberal Party generally advocates economic liberalism (see New Right). Historically, the party has supported a higher degree of economic protectionism and interventionism than it has in recent decades. However, from its foundation the party has identified itself as anti-socialist. Strong opposition to socialism and communism in Australia and abroad was one of its founding principles. The party's founder and longest-serving leader Robert Menzies envisaged that Australia's middle class would form its main constituency. [SEP] Knowledge 2: Australia in the Korean War
The Liberal government of Australia, led by Prime Minister Robert Menzies, immediately responded to the UN resolution by offering military assistance. 17,000 Australians served in the Korean War between 1950 and 1953, and they suffered 339 dead, and 1200 wounded.
Knowledge 1: History of Australia
Politically, Robert Menzies and the Liberal Party of Australia dominated much of the immediate post war era, defeating the Labor government of Ben Chifley in 1949, in part over a Labor proposal to nationalise banks and following a crippling coal strike led by the Australian Communist Party. Menzies became the country's longest-serving Prime Minister and the Liberal party, in coalition with the rural based Country Party, won every federal election until 1972. [SEP] Knowledge 2: Australia in the Korean War
The Liberal government of Australia, led by Prime Minister Robert Menzies, immediately responded to the UN resolution by offering military assistance. 17,000 Australians served in the Korean War between 1950 and 1953, and they suffered 339 dead, and 1200 wounded.
Knowledge 1: History of Australia
Politically, Robert Menzies and the Liberal Party of Australia dominated much of the immediate post war era, defeating the Labor government of Ben Chifley in 1949, in part over a Labor proposal to nationalise banks and following a crippling coal strike led by the Australian Communist Party. Menzies became the country's longest-serving Prime Minister and the Liberal party, in coalition with the rural based Country Party, won every federal election until 1972. [SEP] Knowledge 2: Liberal Party of Australia
The UAP had been formed as a new conservative alliance in 1931, with Labor defector Joseph Lyons as its leader. The stance of Lyons and other Labor rebels against the more radical proposals of the Labor movement to deal the Great Depression had attracted the support of prominent Australian conservatives. With Australia still suffering the effects of the Great Depression, the newly formed party won a landslide victory at the 1931 Election, and the Lyons Government went on to win three consecutive elections. It largely avoided Keynesian pump-priming and pursued a more conservative fiscal policy of debt reduction and balanced budgets as a means of stewarding Australia out of the Depression. Lyons' death in 1939 saw Robert Menzies assume the Prime Ministership on the eve of war. Menzies served as Prime Minister from 1939 to 1941 but resigned as leader of the minority World War II government amidst an unworkable parliamentary majority. The UAP, led by Billy Hughes, disintegrated after suffering a heavy defeat in the 1943 election.

Answer: Lyons' death in 1939