Hindsight is a powerful tool, but not one that people have access to in the present. Unfortunately for history, that means plenty of seemingly brilliant decisions and tactical moves turned out to be legendary blunders. Still, even without a crystal ball, hindsight on some of these seems quite a bit less than 20/20. 

For example, in the late 1950s, Mao Zedong directed the Four Pests Campaign in China to try and increase crop growth. However, Sparrows were one of the four pests selected, and the irradiation of the "pests" caused significant ecological imbalance that wound up negatively affecting the crops. The Great Chinese Famine between 1959 and 1961 followed, yielding the exact opposite results the campaign was trying to create. 

Japan's attack on Pearl Harbor is also one of the biggest military mistakes in history. Incorrectly believing that the harbor housed all of the United States' naval carriers, Japan thought it would be dealing a devastating blow right out of the gates. Instead, it sparked the United States to release its massive naval influence onto World War II, which would lead to Japan's ultimate surrender. 

Check out those, and 23 other massive historical blunders in this gallery of mistakes in hindsight.