Frustrated that his lesser foe to the west was unvanquished, he persuaded himself that the Nazi conquest of Russia would cause Britain to surrender anyway.
"Barbarossa" was launched in July 1941, unleashing the largest invasion force in history, with 4 over million soldiers.
As it was, the troops returned home to a rapturous reception."Wars are not won by evacuation; but there was victory inside this deliverance." So declared Prime Minister Winston Churchill to Parliament, before uttering his most phrases about defending our island, fighting on the beaches, and never surrendering.
The Dunkirk evacuees formed the core of the British army that went on to defeat the Germans in North Africa and later – with American and other allies – returned to the European continent on D-Day in 1944.
The second two mistakes, even more significant in global terms, took place in 1941 and ensured Hitler's eventual defeat as he brought first the USSR and then the USA into the war on Britain's side.
As their French, Dutch and Belgian allies collapsed before the German onslaught, the survivors of the British Expeditionary Force retreated to the channel port of Dunkirk.
Dunkirk represented the first of four massive strategic errors of judgment by Hitler.
Two were committed in 1940, and involved Germany failing to finish off the British when they were down and effectively alone.
On hearing the news, Churchill said he "went to bed and slept the sleep of the saved and thankful". After all, the Japanese attack indeed ensured that the US would go to war – but only against Japan.
Without Hitler's wholly gratuitous declaration, the Roosevelt Administration would never have persuaded Congress to go to war in Europe instead of focusing on the Pacific.