I'm going out on a limb here, but two of my candidates would be Billy Mitchell and Wernher von Braun. Mitchell flew in the face of all his contemporaries by suggesting that air power was how future wars would be won. Nobody in America listened to him, but Hideki Tojo and Hermann Goering certainly paid attention. Marshall was ultimately court-martialed for his defiant insistence that the military listen to his theories, but he got the last laugh - WWII, at least in Europe, was almost certainly decided in the air, and it was a pair of Boeing B-29 Superfortress bombers, Enola Gay and Bock's Car, that ended the war by dropping atomic bombs on Japan. The history of air power in the twentieth century is Mitchell's legacy, the ultimate 'I told you so' from a man who didn't live to see his life's work change the world.
Von Braun was one of the greatest geniuses of the 20th century. He picked up where Robert Goddard and Konstantin Tsiolkovsky left off and developed the most powerful rockets the world had ever seen. Much is made of von Braun's role in the London blitz, but few people realize that von Braun was in many respects much like Oskar Schindler - he employed Jewish workers in his production facilities that would otherwise have gone to the death camps, he never joined the Nazi Party, he stopped just short of designing rockets with enough destructive power to alter the course of the war, and his Peenemunde factory was a model of under-production. In early 1945, von Braun and his team defected to the United States, which made the entire American space program possible. Von Braun and his associates designed and built everything from the eighty-foot Redstone rocket that put the first American satellites and the first American astronaut into space to the 363-foot Saturn V rocket that carried twelve men to the surface of the moon between 1969 and 1972. Before his death, Wernher von Braun had a hand in the development of the space shuttle and early plans for American space stations. Almost every significant American accomplishment in the Space Age was, directly or indirectly, the result of von Braun's work, and no discussion of the twentieth century is complete without him.
