This is a very sticky subject. Obviously the harnassing of nuclear fission as a weapon was a big one. The use of radient energy to locate objects is also a biggie. The jeep was a great idea as well. The ENIGMA encoding device that the Germans created was virtually unbreakable. An increadable construct (recall the ENIGMA code was not broken... a copy was captured by the British).
How about Penicellin? That was probably the greatest invention. The lives it saved is incalcuable.

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Carthage must be destroyed!