As a French, I have to point that the main reasons of the defeat in 1940 were not so social but mainly that everybody, especially in the army, was convinced that French army was the best while it was directed by St-Cyr (french West Point) Generals thinking that horses were the future of the military art and never ever had read anything from De Gaulle, Guderian or Liddle-Hart.
Also, only defensive strategies were established and nobody could even imagine that German panzers could rush through the deep woods of the Ardennes. Maginot line was the unbeatable Fortress.
Aside of this, tanks were great, planes & ships too, and some of the troops also (colonial, Foreign Legion, Corps-francs...) but led by stupid generals and silly tactics.
Regarding what is due to the US, yes, I understand how much it did cost to the average US family to send his sons dying on the far away beaches of Normandy or whatever, and we all feel thankful for this.
Aside of this, if we speak about US politics (and consequently not about the average american) would the US have (as french and british also should have) raised their voice (and maybe more) during the Spanish Civil War, would some industrials not have seen Hitler as a nice guy with whom making business was fun (remember the Ford/Opel trucks and some more...), would the US Congress have been much less isolationist, which makes some people seriously think that Roosevelt made the choice to let Pearl Harbour happen while he was aware of the coming attack, only to have the US finally enter into the war, then maybe a lot more of lives could have been saved.
Of course, America finally did a lot and more for "freedom" and paid its due, but is definitely not the immaculate White Knight and as a rising major power also grabbed a large profit from this conflict...