Two years ago in Prague, U.S. President Barack Obama put forward his visionary idea of a world free of nuclear weapons. A year ago, a new strategic arms treaty between Russia and the United States was signed in the same city. Now the wave of support for a full ban on nuclear weapons is being transformed into a debate about nuclear deterrence. Indeed, the four American strategists who first called for "nuclear zero" — Henry Kissinger, George Shultz, William Perry, and Sam Nunn — have partly backtracked, and are now calling for an end to the doctrine of "mutual assured destruction."