I'd have to go back and re-read the history, but it's been my understanding that part of the delay for Western intervention was political. Was British strategic culture at the time opposed to direct confrontation with a contintental power (I'm thinking of the Napoleonic experience as well)?

Also - I think something like 80% of German casualties were suffered on the Eastern Front. That's a major indicator of who bears the most responsibility for defeating Germany (assuming the Clausewitzian desire of destroying the enemy's capability to fight holds true as the central purpose of military action). Looking at German losses in Barbarossa in 1941 alone, and Soviet mass mobilization, I think Germany lost that winter. Germany was not fully mobilized until late in the war, and while it had the personnel to cover its 1941 losses, even after fully mobilization they never quite reached the replacement rate they had in the early years.

What opportunity did Germany have in 1942 to press for Soviet capitulation that it did not have in 1941? It was the series of defeats in 1942-1943 that finally made the German leadership recognize the trap they were now in.