It's hard to say which was Hitler's greatest blunder. He made so many. Certainly, attacking the Soviet Union proved to be a blunder in time, but when he attacked in June 1941, his troops made it to within 20 miles of Moscow before being stopped by a combination of the Red Army and the onset of winter. Had his troops been properly equipped for the weather, he might well have won.

The Soviet Union murdered at least a million Ukrainians (and some estimates are several times higher) before the start of the war through their forced collectivism policy. When Hitler's troops first entered the Ukraine, they were welcomed as liberators. Quite a few Ukrainians fought for Germany against the Soviet Union. However, Hitler's forces were very brutal, in part because of the large number of Jews living there, and turned the Ukrainians against them. Had they not been so brutal, they probably would've had a lot more support and could possibly have beaten the Soviet Union. But then, stupid brutality against "lesser races" was a cornerstone of Nazism, so being less brutal was against their fundamental nature.

Even earlier in the war, Hitler made a couple huge blunders that ultimately led to his defeat. The first came at Dunkirk when several hundred thousand men of the British armed forced had their backs to the English Channel. Instead of sending in the Wehrmacht to finish them off, Hitler listened to Goring and let the Luftwaffe have the job. As a result, some 300,000 British and allied soldiers were evacuated. Those men were able to fight again another day. The "Miracle at Dunkirk" was a big morale boost for the British. It gave them the additional willpower they would need to withstand the coming Battle of Britain.

Another blunder was how Hitler waited several weeks after the fall of France before beginning the Battle of Britain. This gave the British additional time to organize defenses and to replace some of the planes and pilots lost in France.

Still another blunder during the Battle of Britain was how Hitler ordered the bombing attacks to shift from the RAF to British cities. As hard as it is to believe today, for the first year of the war, both sides pretty well honored prewar agreements to limit aerial bombing to military targets. One night in 1940, a German bomber was lost over Britain (its navigation signals were being jammed). The crew decided to jettison their bombs and fly home. They didn't know they were over London at the time. When they landed, they were ordered to Berlin to face charges of disobeying Hitler's orders. Churchill ordered RAF Bomber Command to hit back, so they sent a small force to bomb Berlin. It actually took a couple tries, but the result was Hitler going berserk. He ordered the bombing shifted from the RAF fields and other military targets to the cities. This ended up killing some 40,000 British civilians but saved their military, especially the RAF which was within a week or two of collapse.

So, not handling the Battle of Britain properly was a monumental blunder. It allowed the Allies a nearby base of operations for bombing attacks (Britain was described as an "unsinkable aircraft carrier") and ultimately as the embarkation point for D-Day. Had Britain been captured, it might not have been possible for us to defeat Hitler.

As to declaring war on America, Churchill wrote in his journal after Pearl Harbor that he "slept the sleep of the saved" because he knew that America's entry into the war was necessary to defeat Hitler. Even though we'd agreed on a Germany First policy months before the war began, the attack on Pearl Harbor channeled America's fury at Japan. When Germany and Italy declared war on the US a few days later (and we returned the favor the same day), it allowed the Germany First policy to proceed. That's when Churchill knew Hitler would lose the war.

Here's an exerpt from Churchill regarding his emotions after hearing of the Pearl Harbor attack:

"Silly people--and there were many, not only in enemy countries--discounted the force of the United States. Some said they were soft, others that they would never be united. They would fool around at a distance. They would never come to grips. Their democracy and system of recurrent elections would paralyze their war effort. They would be just a vague blur on the horizon to friend or foe. That now we should see the weakness of this numerous but remote, wealthy, and talkative people.

But I had studied the American Civil War, fought to the last desperate inch. American blood flowed in my veins. I thought of a remark which Edward Grey had made to me more than thirty years before--that the United States is like "a gigantic boiler. Once the fire is lighted under it there is no limit to the power it can generate." Being saturated and satiated with emotion and sensation, I went to bed and slept the sleep of the saved and thankful."