In the closing days of World War II, scientists working for the U.S. government invented nuclear explosives by splitting the atoms of heavy metals. Germany had already surrendered, but the United States and its allies remained at war with Japan. In the summer of 1945, the Japanese city of Hiroshima was flattened by a single nuclear bomb. A second bombing occurred just a few days later, decimating the city of Nagasaki. These were the first nuclear weapons ever used in war.

And - so far - they are the last.

Since then, tens of thousands of nuclear weapons have been manufactured and deployed by governments around the world. Many of these weapons are much more powerful than the atomic bombs that destroyed the two Japanese cities. None have been used so far, and the absence of nuclear war among nations armed to the teeth with nuclear weapons is a great mystery.

While the threat of a nuclear attack on the United States has receded, the possibility of a nuclear attack on an American city by terrorists has taken its place in our official nightmares. So far, no terrorist group has made a serious effort to buy, steal, or build a nuclear weapon. The absence of nuclear terrorism in a world swarming with fanatical terrorists is another great mystery.

The slippery slope to a nuclear Armageddon has been present for more than sixty years. In secure locations in Washington, Moscow, Beijing, London, and Paris, there are buttons to push than could put an end to human civilization, but these buttons have never been pushed.

Why not? What has so far kept us safe from these mortal dangers?

Those are the questions that Caplow asks and answers in Armageddon Postponed.