Nuclear war
Nuclear bombs were invented by the United States with Canadian and British help during World War II to help make that war stop.  The destruction of Hiroshima and Nagasaki and the Soviet invasion of Manchuria caused Japan to surrender.  They are still the only nuclear weapons that have ever been used in fighting a war.  At that time only the United States had the technology needed to make the bomb, but within a few years the Soviet Union had developed it too. In the new Cold War, the United States and the Soviet Union were enemies and each had many nuclear weapons, but they did not dare to use them against each other, either directly or by attacking the other country's allies. Since either country could be completely destroyed by the other's weapons, nuclear war could no longer be limited to the use of only one or two bombs; if they were used anywhere by one side, the other would attack with its own nuclear weapons, and the fighting would almost certainly become greater. This situation came to be known as the "balance of terror," or Mutually Assured Destruction, and stopped conflicts between the two superpowers from leading to a third world war.