Contrary to the, alas, almost universal misconception, no. The Allies did not win World War II. Nazi Germany (more specifically, Adolf Hitler) lost it. He lost it by making a whole series of strategic blunders starting with his fatal decision in August 1941 not to go directly to Moscow and thus lost invaluable time (almost two months). Dunkirk decision was a blunder as well, but not nearly as fatal.
Hitler fatally overestimated the power of the Red Army which at that time was collapsing just about everywhere (it turned out that overestimating your enemy could be just as fatal as underestimating).
Had he remained loyal to the key concept of his blitzkrieg strategy (risk everything on one decisive blow and win), he would have undoubtedly captured Moscow and – given the political, economic, military and emotional importance of the Soviet capital, won the war on the Eastern front.
As the only way for Great Britain to win the war with Nazi Germany was to bring the Soviet Union into the war on the British side and to transform the blitzkrieg into the war of attrition (and that’s exactly what happened), the defeat of the Soviet Union would have meant for Britain the loss of any hope to win the war. Which would have inevitably led to a peace treaty with Nazi Germany – a clear victory for Adolf Hitler.