Dunkirk was the big mistake. Britain would have been forced to sign a peace treaty if their army couldn't escape and then that would have been the end of the war, before it could spread worldwide. Operation Barbarossa only happened because the Germans were convinced the Soviets were preparing to stab them in the back while they were concentrated on the Western front; but there would be no incentive to do so without a war in the West.
Dunkirk was the big mistake. Britain would have been forced to sign a peace treaty if their army couldn't escape and then that would have been the end of the war, before it could spread worldwide. Operation Barbarossa only happened because the Germans were convinced the Soviets were preparing to stab them in the back while they were concentrated on the Western front; but there would be no incentive to do so without a war in the West.