Final answer:
Winston Churchill was the prime minister of Great Britain for most of World War II, after succeeding Neville Chamberlain in 1940. He was a central figure in the Allied effort and part of the Big Three leaders.
Step-by-step explanation:
The prime minister of Great Britain during most of World War II was Winston Churchill. He rose to the position after the resignation of Neville Chamberlain in 1940 when Britain was in the midst of a critical phase in the war against Germany.
Churchill, a lifelong British politician, had long been wary of Adolf Hitler's rise in Germany, warning against the dangers he posed. During his tenure as Prime Minister, he was known for his stirring speeches and leadership, famously promising his countrymen 'blood, toil, tears, and sweat' in the fight for freedom.
He was a key part of the 'Big Three', along with President Franklin Roosevelt of the United States and Premier Joseph Stalin of the Soviet Union, shaping Allied strategy to defeat Nazi Germany.