Australia had four consecutive Prime Ministers from both sides of politics over a ten year period (Coincidentally, our most intense period of involvement in Vietnam) who had active service in WW2:
Harold Holt (PM Jan '66- Dec '67) was a member of parliament from 1935 to Sep '39, when he resigned to join AIF as an Infantryman. Was recalled to parliament after Dec '40 air accident in Canberra killed 3 Gov senior ministers and senior military officers.
John Gorton (PM Jan '68-Mar '71) was a Pilot Officer flying fighters in RAAF. Wounded in action
3 times
William McMahon (PM Mar '71-Dec'72) enlisted in the 2nd AIF in 1939. He served in the Army throughout World War II, rising to the rank of Major before his discharge in 1945.
Gough Whitlam (PM Dec '72-Nov '75) enlisted in the RAAF 1941, served as a navigator, and was discharged as a Flight Lieutenant in 1945.
Compared with US presidents, it's surprising that none served in the Navy!