Short answers, pulled from the story.
Getúlio Vargas led Brazil as dictator under his Estado Novo regime from 1937 until 1945. He sent the Brazilian Expeditionary Force to Italy where João Baptista Mascarenhas de Morais commanded troops that arrived in 1944.
Winston Churchill became Prime Minister of the United Kingdom on the 10th of May 1940 following Neville Chamberlain's resignation. He served as First Lord of the Admiralty at the outbreak of war and later led Britain through its darkest hours during the Battle of Britain.
Joseph Stalin served as Chairman of the Committee of the People's Commissars starting in 1941 and remained undisputed war leader with supreme authority over the Soviet Union until his death. He led the Red Army to liberate the Soviet Union from Nazi occupation and installed communist leaders throughout Eastern Europe after the war.
By war's end the British Indian Army had grown from 183,000 men in 1939 to 2,250,000 soldiers. Muhammad Ali Jinnah asked all Indians to join the British Army against Nazi Germany while Archibald Wavell commanded Middle East forces early in the war before Claude Auchinleck took over in 1941.
Douglas MacArthur accepted Japan's surrender on the 2nd of September 1945 then became Supreme Commander Allied Powers overseeing occupation duties. He served as Supreme Commander of Allied Forces in the South West Pacific Area from 1942 until 1945 and commanded US and Filipino forces in the Philippines before relocating to Australia.