[Franek Grabowski]

Well, I would say there is no single breaking point that turned the course of war. The Big Week was not a breaking point itself, it was just a step in the right direction. Another one was airwar for supremacy over Normandy, which resulted in virtual annihilation of several Luftwaffe units to the degree they were unable to recuperate. Still, Luftwaffe existed, but it was existence to cease. Mustang was just only a factor, but a major factor because it was the aircraft able to reach almost every German target and still be competitive with any aircraft Germans could have produced. This was a key factor for the Big Week but also for Normandy.
I must disagree with some comments concerning Luftwaffe though. Most of the problems were based on inadequate training of the personnel, and resulting inability to fight against large formations of aircraft. Also a very poor navigation training, which resulted with many units being unable to arrive to Normandy within expected time. It is noticeable that there were several airmen arriving to units in Normandy, who were trained for about two years but still not capable of flying combat missions. In my opinion, part of the credit for victory should go to the Luftwaffe itself.
And just a few words about this old Soviet myth of second front. It did exist well before Soviets switched sides in Balkans or in Africa. It continued to exist with the invasion of Italy in 1943. It was over Germany, in Burma, New Guinea and Pacific. It was in occupied Europe. This was a World War not without reason.