13. The German A-Bomb
Many believe that if WWII had lasted just a little longer, the Germans would have won or at least secured a draw, because they had been on the verge of developing an atomic bomb when the Third Reich crumbled. The Germans were actually never close to developing a nuke. During the war, American and British scientists assumed that Germany had an advanced nuclear program that might bear fruit at any time. They thus saw themselves as being in a race to produce nuclear weapons.
After the war, however, it turned out that the German nuclear program was nowhere near as advanced as previously thought. Early in their research, German physicists took a wrong turn, and followed it down a path that led away from atomic fission. The war could have lasted another decade, and Germany would have been no closer to having an atomic bomb in 1955 than she had been in 1945.
Related: World War II Race for Nuclear Weapons.