“The first was to secure uninterrupted control of the sea-lanes supplying the British Isles, the great future base for the Allied invasion forces, but that had meant the defeat of the U-boat menace, which came only in the summer of 1943. The second was to attain command of the air, not just over France but also over the Reich, and that was properly secured, interestingly enough, only by early 1944.23 The third and absolutely critical prerequisite had been of course the entry of the United States into the war and the commitment by the American government to a Germany First strategy, for only its vast productive power could guarantee that the Allies would be strong enough to push their way into France. Yet there was also a fourth great factor at work, though it was far to the east, namely, the vast Nazi-Soviet struggle that sapped so much of the Third Reich’s resources and was still pinning down the majority of the German Army’s divisions in 1944.”