It was a huge blunder. Was it a bigger blunder than Operation Barbarossa?
If you consider that Germany’s attack on the Soviet Union ultimately led to Germany’s defeat, then a look at war casualties offers a comparison:
https://en.wikipedia.org/wiki/World_War_II_casualties
The Japanese lost 3 million out of a population of 71 million (about 4%)
Germany lost 7 million out a 69 million population (about 10%)
The Soviet Union lost 24 million out of 188 million population (13%)
America lost 419,000 people out of 131 million ( about 0.2%)
The US wasn’t in the war per se until Pearl Harbor, and after the Japanese attacked, people were lined up around the block to enlist.
Lend-Lease helped both the UK and the USSR, but actually fighting the Germans (even through Churchill’s “soft underbelly”) helped tip the scale. Hitler went to direct Zitadelle (Kursk) in person, and shifted three divisions out of the preparations for the battle line to shore up Italian defenses. Boneheaded.
The terrain of Italy was the defense. And after those dozen or so airfields in southern Italy were in allied hands, the US finally put its foot down with the British and said, we’re done trying to fight our way up this BS, we have our objectives in hand.
Not to mention, the full industrial mobilization happened, and would not have otherwise, leading to overwhelming firepower from us and our allies.
Compared to the Germans, the Japanese were midget rodeo clowns (no ethnic slur intended, but I stand by the midget thing), and we had to do most of that on our own, while still making Germany-first our priority.
Hitler’s mistake vs the USSR was just one of a series of errors, the largest one. Obviously he should have cut loose and bagged the British army at Dunkirk. Failing that, should not have started Barbarossa until putting adequate resources to finish off the British in North Africa, there’s no doubt they could have prevailed if he had. Imagine that, the Med a German lake, control of the Suez Canal, plenty of oil from Arabia, and still no new front to worry about.
Instead, he fought the war like it was a bus schedule, and fretted that they couldn’t possibly keep the timetable if he had to stop to pick up passengers. Didn’t grasp strategy or tactics.
Compared with Pearl Harbor, well, nothing compares to it.
But how much of that was due to Stalin's egoism in slaying so many commanders, and not preparing soon enough for war?
"America lost 419,000 people out of 131 million ( about 0.2%)"
By the grace of God, part of which is the "ocean effect."
But today we could not fight WW2 on its terms, as China could. How ridiculous was American production in World War 2? [images at link]
Operation Barbarossa was technically, tactically and strategically perfect.
The problem afterward was twofold.
Firstly, Hitler wanted Ukraine and Lebensraum, splitting the forces which should have went straight to Moscow.
The secondary problem was the immediate implementation of the SS in those countries Germany took over. At the beginning, those countries looked at German troops as saviors from the USSR and would have gladly supported the Germans and fought against Stalin. The SS soured this massively and put a huge partisan movement over their supply lines.