EVERY TIME Barack Obama announces that he is in favor of a world free of nuclear weapons, the nuclear hawks descend. Soon after his inauguration, former–Reagan administration Pentagon official Frank Gaffney proclaimed that the president “stands to transform the ‘world’s only superpower’ into a nuclear impotent.” After Obama promised in his 2009 Prague speech that “the United States will take concrete steps toward a world without nuclear weapons,” former–Secretary of Defense James Schlesinger declared that “the notion that we can abolish nuclear weapons reflects on a combination of American utopianism and American parochialism.” And when the president won the 2009 Nobel Peace Prize, in part for his embrace of the disarmament vision, Time Magazine even ran an essay entitled “Want Peace? Give a Nuke the Nobel.”

Obama is right to declare, loudly and often, that the United States seeks a world without nuclear weapons, and the administration is right to be taking concrete steps now toward that long-term goal. Indeed, by proclaiming that America seeks nuclear zero, Obama is simply reaffirming that we will follow our treaty commitments: states that joined the 1970 Nuclear Non-Proliferation Treaty (NPT) agreed “to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament.” And since Article 6 of America’s Constitution says that a treaty commitment is “the supreme Law of the Land,” at a basic level, Obama is simply saying that he will follow U.S. law.

The abolition aspiration is not, however, based on such legal niceties. Instead, it is inspired by two important insights about the global nuclear future. First, the most dangerous nuclear threats to the United States today and on the horizon are from terrorists and potential new nuclear powers, not from our traditional Cold War adversaries in Russia and China. Second, the spread of nuclear weapons to new states, and indirectly to terrorist organizations, will be made less likely if the United States and other nuclear-armed nations are seen to be working in good faith toward disarmament.

Nuclear weapons may have been a dangerous necessity to keep the Cold War cold. But scholars and policy makers who are nostalgic for the brutal simplicity of that era’s nuclear deterrence do not understand how much the world has changed. The choice we face is not between a nuclear-free world or a return to bipolar Cold War deterrence; it is between creating a nuclear-weapons-free world or living in a world with many more nuclear-weapons states. And if there are more nuclear nations, and more atomic weapons in global arsenals, there will be more opportunities for terrorists to steal or buy the bomb.

THE THREAT of nuclear-armed terrorists is not new. In 1977, the Red Army Faction in West Germany attacked a U.S. military base hoping to steal the tactical nuclear weapons there. The Aum Shinrikyo apocalyptic cult in Japan sought recruits in the Russian military in the 1990s to get access to loose nukes and only settled on using sarin-gas chemicals in the Tokyo subway when their nuclear efforts failed. Today’s threat is even more alarming. It is well known that Osama bin Laden has proclaimed that Islamic jihadis have a duty to acquire and to use nuclear weapons against the West. And al-Qaeda is known to have recruited senior Pakistani nuclear scientists in the past and may now have “sleeper agents” in Pakistani laboratories to help in that effort.

The easier-to-acquire radioactive dirty bomb with its concomitant threat to kill up to one thousand people and create environmental havoc is already a reality. In 2004, Dhiren Barot, a veteran of jihadi campaigns in Kashmir, was arrested in London. He admitted to plotting attacks against the New York Stock Exchange and the World Bank and possessed detailed plans to acquire nuclear materials from ten thousand smoke detectors for a radiological device. In a report sent to al-Qaeda central, Barot wrote that “estimated casualties [would] be in [the] region of 500 long-term affected if dispersed in [a] busy area (Inshalla).” A homegrown dirty-bomb threat has also emerged: in 2009, James Cummings, a neo-Nazi in Belfast, Maine, was discovered to have started collecting low-level nuclear materials.

The even-more-destructive terrorist-nuclear-weapons danger is looming on the horizon. Terrorists are not likely to be deterred by threats of retaliation. Stopping them from purchasing a nuclear weapon, or stealing one, or getting the materials to make their own is a much better strategy. If aspiring nuclear-weapons states—such as Iran and Syria (and some suspect Burma)—get nuclear weapons in the future, the danger that terrorists will get their hands on one will clearly increase. And if the United States and other nuclear-weapons nations are seen to be hypocritical, by not following our NPT commitments and maintaining that we (but only we) are responsible enough to have them, it will reduce the likelihood of ensuring the broad international cooperation that is needed to reduce these proliferation risks.

OFFICIALS IN the George W. Bush administration believed that there was no link between U.S. arsenal size or military posture and nonproliferation decisions made by non-nuclear-weapons states. The Obama administration’s new Nuclear Posture Review maintains that the connection is strong, even if it is often indirect and hard to measure:

***By demonstrating that we take seriously our NPT obligation to pursue nuclear disarmament, we strengthen our ability to mobilize broad international support for the measures needed to reinforce the non-proliferation regime and secure nuclear materials worldwide.***

There are now many signs that the Obama administration is correct in its assessment that progress in disarmament enables progress in nonproliferation. The April 2010 Nuclear Security Summit brought forty-six countries to Washington where they reached agreement on a number of concrete steps to better protect nuclear materials from terrorists. And in stark contrast to the Bush-era 2005 NPT Review Conference, which ended in failure, the May 2010 review took place in a cooperative atmosphere and produced a final document that called on all states to sign onto improved safeguards for their reactors, and encouraged governments not in compliance with their treaty commitments to change their ways. The successful efforts to get additional rounds of sanctions against Iran in the UN Security Council can be credited, in part, to the new spirit of cooperation, including the progress on arms-control agreements between the United States and China, and the United States and Russia.

SEVERE CHALLENGES to global zero remain. It will be critical that all states have increased confidence that final disarmament agreements will be enforced and that any new nuclear proliferator will not be tolerated. Fortunately, in a nuclear-free world, the former nuclear-weapons states would have far stronger mutual incentives to punish and reverse any new state’s decision to acquire atomic bombs. Ironically, it is precisely because nuclear-weapons states have such large arsenals today that they sometimes succumb to the temptation to accept new proliferators. In a disarmed world, such complacency would be more obviously imprudent, thus encouraging the once-nuclear-armed states to enforce nonproliferation.

Verification at zero (or at low numbers for that matter) is an obvious challenge. Even if better verification technology is created, there will remain the problem of what to do if an erstwhile nuclear nation is caught secretly preparing to rearm. A way around this is to accept the fact that all former nuclear-weapons states will retain the option of reversing course. Ironically, this capability will be both reassuring and deterring: reassuring because it enables states to begin taking the final steps toward total nuclear disarmament even in the absence of complete confidence that the process will be successful; deterring because each state will know that even if it can reverse its final disarmament steps, so can the others. In short, there will still be a latent form of nuclear deterrence even in a nuclear-disarmed world.

Finally, there is the question of ballistic-missile defenses. During and immediately after the Cold War, many saw these systems as “destabilizing” because as long as a government’s nuclear security was dependent on the ability to retaliate with devastating force after an attack, if an adversary hitting first could use even limited defenses to reduce the effectiveness of second-strike retaliation, mutually assured destruction no longer held. Managed mutual-missile-defense deployments in the future could, however, permit the final steps of disarmament to take place with less concern about cheating in the immediate term and could provide more confidence in the ability of governments to respond in a timely manner to successful rule breaking by another state.

THE NUCLEAR-WEAPONS-FREE world will not be a world free of conflicts of national interest; nor will it be a utopia in which governments never feel tempted to cheat on their international obligations. A world without nuclear weapons will not be a world without war. Indeed, the maintenance of global zero will require that conventionally armed major powers be prepared to enforce nuclear disarmament and nonproliferation commitments in a fair and vigorous manner. Potential proliferators may have to be “forced” to be free.

In medieval times, European mapmakers placed the words hic sunt dracones (here be dragons) at the edge of the known world. Disarmament critics today are like those medieval mapmakers, fearing that we are entering unknown territory fraught with hidden nuclear monsters. But these dragons are fantasies. The genuine strategic challenges we face in creating a secure nuclear-free world—adequate verification, enforcement of violations and mutual-defense deployments—are challenges that can be met over time. And the world we are heading toward if we fail to find safe paths to mutual and verifiable disarmament—a world crowded with nuclear-weapons states and terrorist temptations—is even more fraught with danger.