Of course, we had a world without nuclear weapons not so long ago -- say, in 1939. The war that began in that nuclear-free world led to a crash project to develop nuclear weapons. It ended with America's use of them -- something Obama alluded to: "As a nuclear power, as the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act."

It is not clear whether this statement implies disapproval of our use of nuclear weapons in 1945. It's telling, however, that Obama never referred in his Prague speech to the Second World War. Instead, he called the existence of thousands of nuclear weapons "the most dangerous legacy of the Cold War." This framework makes it possible to think of the elimination of nuclear weapons as a logical response to the end of that conflict: "Today, the Cold War has disappeared but thousands of those weapons have not."