National and international security problems are important for one reason above all others: their human consequences are staggering. In the twentieth century, armed conflicts killed tens of millions of people, wounded tens of millions more, and drove tens of millions of people from their homes. In addition, the economic costs of security problems—the costs of defense preparations, the costs of wartime military operations, and the costs of postconflict reconstruction—have been enormous. Many security problems of the twentieth century will persist in the twenty-first, some will evolve and become deadlier than ever, and new problems will be added to the security agenda. National and international security will be momentous policy problems for the foreseeable future.
A brief review of security problems in the twentieth century brings this picture into sharper focus. During World War I more than 8 million soldiers were killed, more than 6.6 million civilians were killed, and more than 21 million soldiers were wounded. Over the course of World War II in Asia and Europe, an estimated 15 million soldiers were killed, at least 26 million and perhaps as many as 34 million civilians were killed, and at least 25 million soldiers were wounded.
The decades after 1945 have been referred to as "the long peace" because the great powers of that era—the United States and the Soviet Union—did not engage in allout military hostilities even though they had an alarming number of opportunities to do so. One of the most dangerous aspects of the cold war was the nuclear arms race between the United States and the Soviet Union. Between 1945 and 1990 the two superpowers built and deployed tens of thousands of nuclear weapons. At the peak of the arms race in the 1980s, their nuclear arsenals contained approximately 24,000 strategic nuclear weapons deployed on an array of intercontinental-range delivery systems and at least 23,000 tactical nuclear weapons deployed on land, sea, and air around the world. Most of these weapons were many times more powerful than the atomic bombs that were used in the U.S. attacks on Hiroshima and Nagasaki in 1945; some strategic weapons had explosive yields one to two thousand times greater than the bombs dropped on Japan.
There were two main schools of thought about the likely consequences of an all-out nuclear war between the United States and the Soviet Union. Some believed that a war involving tens of thousands of nuclear weapons would kill billions of people, obliterate all of the world's leading powers, and end civilization as we knew it. These were the optimists. The pessimists believed that tens of thousands of . . .