From allowing the rise of Germany in World War II to putting lead in gasoline and asbestos in walls, humanity has made some pretty bad decisions over the years. But hopefully, if we're aware of what we messed up in the past, so we can make better decisions in the future. 

All the way back in the 13th century, the head of the Khwarezm empire made a decision so poor that we're still experiencing the repercussions of it today. Learning from an envoy that one of his governors had killed a neighboring territory's peaceful trade delegation, he decided to kill the envoy too instead of administering justice. It just so happened that the head of the neighboring territory was Genghis Khan, who decided to turn his attention away from early China and the east, and toward this new disrespectful enemy to the West. Of course what followed was arguably the biggest militaristic conquest in history, as Genghis vanquished everyone in his path to majority control of Asia and Eastern Europe, forever shaping the formation of the modern world. Can you think of a worse decision than that?

You can read about 22 other contenders here, and be the judge yourself.