One, guns. Before guns, people were still stabbing each other with swords and spears, and warfare was a lot more quiet. Guns are like killing MACHINES compared to swords and co., so much so as that they have nearly completely overcome the weapon industry. Two, the atomic bomb. The atomic bomb allowed people to create mass destruction in a small amount of time. In fact, (theoretically speaking) even someone as fast as the flash could not outrun an atomic bomb. It also makes an area unable to inhabit any sort of life for an extremely long time, because of the lasting effects of radiation.