U.S. President Barack Obama has committed his administration to the long-cherished dream of a world without nuclear weapons. In a landmark speech in Prague last weekend, Mr. Obama pledged that the United States would demonstrate “moral responsibility” and lead international efforts toward that goal. It will be a long and difficult task, not least because many believe such ambitions are naive and dangerous. Mr. Obama must not bow to that sentiment. Overcoming such deeply ingrained beliefs will require efforts by all nations. The key to realizing a world without nuclear weapons is ensuring stability by other means.

At the end of the Cold War there was a collective sigh of relief as the threat of mutually assured destruction dissipated. Many people anticipated the lifting of the nuclear shadow that had engulfed humankind. They were wrong. The prospect of superpower confrontation ended, but tens of thousands of warheads and thousands of tons of nuclear materials remained. As Mr. Obama noted, “in a strange turn of history, the threat of global nuclear war has gone down, but the risk of a nuclear attack has gone up.” Nuclear technology and knowhow has spread and there are ample supplies of the materials that can be used to build bombs.

In his Prague speech, Mr. Obama was unequivocal: “I state clearly and with conviction America’s commitment to seek the peace and security of a world without nuclear weapons.” In concrete terms, that means that he will seek U.S. ratification of the Comprehensive Test Ban Treaty (CTBT), which was signed by former U.S. President Bill Clinton but never passed the Senate; convene a summit to halt the spread of nuclear materials; develop a treaty that halts the production of fissile materials; and create an international nuclear fuel bank so countries can use nuclear technology for peaceful uses, as is their right. As a first step, the U.S. and Russia will begin negotiations on a new strategic arms reduction treaty and bring the other nuclear-weapon states into those talks.

The speech could not have been more timely. It was just preceded by North Korea’s launch of a multistage rocket, a critical step in Pyongyang’s quest to put nuclear warheads on long-range missiles. Just over the horizon is the 2010 Review Conference of the Nuclear Nonproliferation Treaty (NPT), a cornerstone of the global nonproliferation regime. The NPT is in danger of collapse after North Korea’s withdrawal, Iran’s game of cat and mouse with nuclear inspectors, and the failure of the nuclear-weapon states — the U.S., Russia, France, Britain and China — to take their disarmament obligations more seriously.

The last review conference, held in May 2005, was a failure. A second consecutive failure could lead to the unraveling of the treaty. To avoid that end, Mr. Obama has said that the U.S. will provide more resources and authority for international inspections and mandate “real and immediate consequences” for countries that violate the treaty.

But it is impossible to prevent nuclear proliferation when nuclear-weapon states are not sending out the right signal. The modernization of nuclear arsenals, the integration of those weapons into defense planning, and the association of nuclear weapons with status all tell nonnuclear states that possessing weapons of mass destruction actually makes sense.

Mr. Obama is under no illusions about the difficulty of his task. Distasteful though it may be, WMD provide the backbone of security policy, not only for their possessors, but for their allies as well — including Japan. Ironically, the prospect of a U.S. administration embracing genuine disarmament — a pillar of Japan’s postwar diplomacy — raises questions about Japan’s own defense. Thus, Mr. Obama insists that his commitment to reduce weapons would proceed in parallel with the long-standing commitment to provide defense for the U.S. and its allies and deterrence against their adversaries.

The key to real progress toward a nuclear-free world is the development of a more effective security framework that does not rely on nuclear weapons. That means countries that possess nuclear weapons must develop defense doctrines that downplay their use; Mr. Obama has made this one of his first priorities. In addition, there may be a need to develop defense systems that can prevent nuclear attacks. Many countries, and not just the U.S. and its allies, are moving forward with such plans.

Equally important is the development of diplomatic mechanisms that can defuse conflicts before they break out and multilateral institutions that can take firm action if needed. No such mechanisms exist, the United Nations notwithstanding. Creating them and making them work is a shared task of every nation that seeks a nuclear-free world.

Some will say this goal is impossible: Knowledge cannot be unlearned and the nuclear genie cannot be returned to the bottle. That may be true, but there are ways to prevent nations from wanting or developing those weapons. It will not be easy, and it will take time. But nuclear danger is too great to ignore.

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