“So today, I state clearly and with conviction America’s commitment to seek the peace and security of a world without nuclear weapons,” President Obama said at the open-air rally in Prague on April 5. With these words came a change in the global air, as if a window had been opened a crack in a dark room that had been sealed shut for decades. On only two previous occasions had an American president proposed the abolition of nuclear arms. The first was Truman’s proposal at the United Nations in 1946 to place all nuclear technology under international control and devote it entirely to peaceful purposes, and so to strangle the nuclear age in its cradle. Stalin’s Soviet Union, bent on developing the bomb, would not agree.

The second was the summit meeting at Reykjavik, Iceland, in 1986, where President Reagan and General Secretary Gorbachev came within an ace of agreeing to full nuclear disarmament. Their bid foundered on Reagan’s Strategic Defense Initiative, which he would not give up and Gorbachev would not accept. Thereafter the pronuclear consensus was restored. Its chief assumption, embodied in the doctrine of deterrence, was that safety from nuclear weapons paradoxically depended on their continued presence. Unremitting readiness to carry out genocide and worse had somehow been accepted as an inescapable commitment of even the greatest civilizations.

Obama’s words disrupted this collective suicidal trance. He placed his commitment in an appropriate context: Prague had been the scene of Czech protests against Soviet domination, and Obama saluted those “who helped bring down a nuclear-armed empire without firing a shot.” The reference was doubly fitting. In the first place, the popular movement broke the spell of omnipotence that had surrounded the totalitarian empire. Like the bomb, the Soviet Union had been shielded by a reputation of immovability. The resistance movements in Czechoslovakia and elsewhere, using the “power of the powerless,” in the phrase of Václav Havel, gave the lie to this illusion. They revealed the possibility of “the impossible” and made it happen. Obama acknowledged the parallel with nuclear disarmament when he took note of those “who hear talk of a world without nuclear weapons and doubt whether it is worth setting a goal that seems impossible to achieve,” and, advising Czechs to remember the lessons of their Velvet Revolution, declared fatalism “a deadly adversary.”

In the second place, it was that same resistance, together with Gorbachev’s perestroika, that by ending the cold war opened the clearest path to nuclear disarmament since 1946. Now that the rivalry that had been used to justify the threat of annihilation had been liquidated, might it be possible to eliminate the weapons that posed that threat? Might this “impossible” thing also be possible? The first three post-cold war presidents passed up the opportunity. Obama has seized it.

Unfortunately, as soon as he announced the goal of abolition, he added that it would not “be achieved quickly, perhaps not in my lifetime.” With those words, the crack of the window seemed to narrow, the moral gloom thickened and the fatalism he had just renounced settled in again. Sighs of relief were almost audible among the upholders of the pronuclear consensus. As The Economist noted, “The world may never get to zero. But it would help make things a lot safer along the way if others act in concert. If North Korea and Iran can keep counting on the protection of China and Russia in their rule-breaking, progress will be all too slight.” In other words, a likely insincere commitment to abolition is to be a new talking point in stopping others from joining the nuclear club, which, for its part, will go on as before.

A further sentence in Obama’s speech gave support to such views. Speaking of the Nuclear Non-proliferation Treaty (NPT), the president said, “The basic bargain is sound: countries with nuclear weapons will move toward disarmament, countries without nuclear weapons will not acquire them.” But moving toward disarmament is not the same as disarming. It is one thing to say to the world, “We all must do without nuclear weapons,” and quite another to say, “You must do without nuclear weapons, and we will keep 1,500 of them for as long as we are all alive.” In the latter case, the abolition commitment would become one more layer of hypocrisy in a situation already overloaded with it. But after more than sixty years of deceptive promises, the countries that do without nuclear weapons will not accept a “bargain” that gives a new lease on life to a double standard they already reject.

These fears are mitigated by the agenda of measures Obama announced as first steps toward abolition. A wish list of arms controllers of recent years, they include ratifying the Comprehensive Test Ban Treaty; negotiating a fissile material cutoff treaty; negotiating mutual cuts in nuclear warheads with Russia, perhaps to a level of 1,500 or 1,000; and fortifying the NPT. These proposals would be welcome in any context, but they take on added meaning when viewed as way stations on a journey to a nuclear-weapons-free world. Most interesting, perhaps, was Obama’s promise to host a Global Summit on Nuclear Security in the next year. Will it concentrate solely on nonproliferation or acknowledge the indispensable link between that goal and full nuclear disarmament? The answer, of course, will not depend on Obama alone. He has brought the nuclear dilemma back into public view. But his vision is a work in progress, a ground of contention on which all who desire disarmament are invited to exert themselves.

Was Obama’s speech historic? Not yet. It was an invitation to participate in history. It will be historic if we make it so. Obama says he is prepared to postpone abolition until he has died. He is 47. I wish him long life. Let us free the world of nuclear weapons while he is still among us.