Airborne RADAR was a great development, but in terms of the war, I think the British giving the USA RADAR combined with ASDIC (which we renamed SONAR) was death on Axis shipping and allowed Allied shipping across the Atlantic to limp along until the British cracked the Enigma code. Once Enigma was cracked, the combination of RADAR, SONAR and aircraft carriers escorting convoys was the death of the wolfpack system. With the wolfpack relegated to shore defense the USA and Canada could use their huge shipping capacity to supply the USSR on Germany's Eastern Front, the allied effort in Europe, and the allied and Chinese efforts in the Pacific.

There was no single scientific or engineering advance that was a keystone. More than anything else it was British desperation that drove them to trust the United States, and our living-up to that trust, that sealed the fate of the Axis powers. Without that reciprocity, the USSR would not have been resupplied and Barbarossa might well have succeeded despite the magnificent sacrifices of the Soviet people.

B

http://www.daileyint.com/seawar/metalog.htm