The dropping of nuclear bombs on Hiroshima and Nagasaki ended World War II but ushered in an entirely new form of conflict that came to be called the Cold War. During the Cold War, the United States and the Soviet Union each built up enormous arsenals of nuclear weapons designed to deter the other from launching a conventional military or nuclear attack. At the time, deterrence worked in the sense that the United States and the Soviet Union did not come into direct military conflict with each other. But these vast nuclear arsenals did not deter the Soviets from using conventional military force in Czechoslovakia, Hungary, or Afghanistan. They did not deter the United States from using military force in Korea and Vietnam. And they did not preclude both sides from amassing large conventional forces in Europe.
When the Cold War ended, many hoped that a new era of peace would replace the threat of large-scale nuclear war breaking out at any moment. Many believed that this peace would be accompanied by a significant global reduction in nuclear weapons. Instead, new challenges to world security arose. Regional instabilities led to threats of war between India and Pakistan, on the Korean Peninsula, and in the Mideast. These threats contributed to and were exacerbated by the proliferation of nuclear weapons in these regions. Additionally, catastrophic terrorism arose as a new threat to world security, with large-scale attacks on civilian populations in the United States, Russia, India, . . .