Owen, I'm sorry but...
The Mustang was developped from a RAF demand, they requested specifications that were far away from USAAF ones. Originally, USAAF didn't want to hear about this aircraft: P47 and P38 only.
The fact that the aircraft had a so long range is a hasard, they created an aerodynamic effect they will not discover before the end of the war.
And, without the Merlin engine, this great fighter would never have been able to play the role he did.
Winning a war is 90% a matter of logistics.
Once USA was in the war, it opened nearly unlimited ressources for the allies and none of the axis nations were able to bombed them, the US industry was unstoppable. This is why having USA into the war was so decisive.
My opinion regarding WWII is that the real decisives battles were fought in North Africa in 1941. Commonwealth didn't lost Egypt and kept a foot in Mediterranean area, that's the key.
Then, USA entered the war and landings one year later in november 1942 were performed with new escort carriers that were started to be produced at many units per month, something that England was not able to do. Just an exemple.
From this point, and with the giant Russian industry that was producing more and more war machines, it was the end.
Then, it was a matter of strategy in order to find the best way to end this horror as quickly as possible.
I'm not sure that Germany wanted or could attempt an invasion of England in the summer on 1940.
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