In the late 1940s, during the brief period when the United States had atomic weapons and the Soviet Union did not, Robert Oppenheimer, director of the Manhattan Project, had a conversation with President Harry S. Truman:

"When will the Russians be able to build the bomb?" Truman asked.

"I don't know," Oppenheimer said.

"I know," the president said.

"When?"

"Never."

Truman was wrong, of course. He was determined to keep America's nuclear advantage, but knowledge cannot be forever sealed in a safe somewhere in the White House basement. It wasn't true in 1949, when the Soviet Union exploded its first atom bomb and kicked off the Cold War, and it is even less true today, when the globe is so electronically interconnected that information can circle the Earth instantaneously. Detailed knowledge of how to build a nuclear weapon is already widespread. The genie has been out of the bottle for decades.

Yet somehow we persist in believing the opposite. Just weeks ago, President Bush famously remarked, "I've told people that, if you're interested in avoiding World War III, it seems like you ought to be interested in preventing (Iran) from having the knowledge necessary to make a nuclear weapon."

The WWIII mention got the most attention, but it was the last part of his comment that was the most startling to me.

It is possible to impede the transportation of enriched uranium and plutonium, the key ingredients, and it is also possible — as Israel has demonstrated — to set back the construction of reactors, enrichment facilities, and other industrial plants necessary for the development of a nuclear arsenal. But in the long run, powerful states like Iran that feel their national security hinges on the bomb will figure out a way to build one. It makes sense to use international leverage to dissuade Iran, and it may even make sense down the road to use military force to slow its progress — but no matter how hard we wish it, we cannot put the nuclear secret back in the safe.

It was never ours to keep.

The physics behind the bomb came from an international scientific network. During the war, owing partly to the justness of the Allied cause, partly to the stupidity and murderous anti-Semitism of Hitler, some of the greatest scientists in the field emigrated to the United States, and to Oppenheimer. Intellectually, the bomb was an international production.

Today's network of nuclear knowledge, in the words of William Lange-wiesche in "The Atomic Bazaar," is "not a rigid structure that can be shattered and shut down, but something rather more like the Internet — a vast, informal web of infinitely flexible connections, capable of reshaping itself with ease, and concerned primarily with the flow of knowledge. Einstein and Oppenheimer predicted this years ago, when they warned that there are no secrets to protect."

This means that the spread of knowledge, even nuclear knowledge, is inevitable. In the future, many more small nations will have nuclear weapons, and the likelihood that they will be used will grow.

This is terrible news, but it hardly compares to the jeopardy the planet faced during the Cold War, when the launching of a single nuclear missile could have triggered a catastrophic U.S.-Soviet exchange. If Pakistan and Iran were foolish enough to lob nukes at each other, millions would die and the two countries would both effectively cease to exist. The rest of us would have to deal with radioactive fallout and a horrific humanitarian crisis. Let me say again: This is a dreadful thing to contemplate, but it is not (with all deference to Bush) World War III, nor is it ever likely to even come close.

Does Bush stop to think how arrogant the words preventing them from having the knowledge sound in the rest of the world? He controls the planet's largest nuclear arsenal, well in excess of 10,000 warheads, and leads the only nation to have ever used atomic weapons in war. In the words of Langewiesche, the Nuclear Non-Proliferation Treaty, which seeks to stop the spread of nukes, also "enshrines" a "discriminatory nuclear order." It says, in essence, that the existing nuclear powers are entitled to have them, but the rest of the world is not. This is not an argument likely to persuade the have-nots.

Once we accept that, in the long run, we cannot prevent the spread of nukes, voluntary disarmament is the only sensible public policy. Only in a world in which everyone is aggressively disarming would the logic of voluntary disarmament be persuasive to the powerless. It must proceed out of a universal acknowledgment that the weapons are simply too destructive — that no nation or cause will likely survive their use. That means the great powers must be determined not just to prevent the construction of new nuclear weapons, but also to get rid of the old ones. The United States and Russia are dismantling their overkill Cold War arsenals, but not nearly fast enough. At the same time, both countries are undermining the disarmament cause by testing and building new generations of such weapons.

With U.S. forces occupying Iraq and Afghanistan, and with a nuclear Pakistan on Iran's southeastern border, the Islamist Republic is surrounded by nuclear-armed adversaries. When I visited Tehran several years ago, even moderate pro-West Iranians who could read a map supported development of their own nuclear deterrent. They scoffed at the idea that they should be prevented from doing so by the United States or anyone else, just as Americans would scoff at the idea that Russia should dictate the terms of our own security.

"What hypocrisy!" one friendly Iranian journalist remarked to me. I suspect the bellicose policies of the Bush administration have done little to dampen this point of view, and have done much to strengthen the hand of the religious radicals in charge.

If ever Theodore Roosevelt's advice — "Speak softly and carry a big stick" — made sense, it does so today in our dealings with Iran. The outcome we should most desire is for the mullahs to conclude that the cost of building a handful of nukes is too great, and that the advantage in having them is too small. The more Iran feels threatened, the less likely that becomes.

Mark Bowden is author of "Guests of the Ayatollah: The First Battle in America's War with Militant Islam."