As a young man Winston Churchill was thought to be mentally slow. He did not do well in school, but he got into good schools and gained positions because he was the son of an English Lord.
After his military service in World War I, he held a series of Cabinet posts. He then engaged in politics by running for the House of Commons. Sometimes he won; sometimes he lost. In short, he had an undistinguished political career.
As Adolph Hitler came to power in Germany, Churchill became a vocal opponent of England's appeasement policy. He warned about the looming German threat. (Prime Minister Neville Chamberlain was determined to pacify the German dictator. He feared opposing Hitler would bring England to the brink of war.) At first Churchill's views were unpopular, but by 1940 he was elected Prime Minister.
It was Winston Churchill, the not too smart school boy, more than any other factor that held Great Britain together during World War II.