ASHINGTON — Not so long ago, the terrifying rules of nuclear chicken were clear.

When only superpowers and their allies held nuclear arsenals, deterrence worked, because all sides understood the horrific consequences of a misstep. Even during the most unnerving confrontations, like the Cuban missile crisis, there were clear "red lines" beyond which no sane leader would intentionally step. And as nuclear technology spread, new red lines emerged. Israel enforced one 24 years ago, when it destroyed Saddam Hussein's nuclear reactor rather than let him get near a bomb.

But the lesson of the past few years is that red lines have blurred, to the point where they are now little more than pink smudges. And now, no one seems to know the rules. Not the Bush administration, as it sends conflicting signals about what it and its allies will do if diplomacy fails to disarm Iran and North Korea. Not Kim Jong Il, or the Iranian mullahs, as they test new and undefined limits. And why not test them?

They all know that India, Pakistan and Israel joined the nuclear club without ever accepting the rules laid out in the Nuclear Nonproliferation Treaty. Even after India and Pakistan set off tests in 1998, the sanctions America imposed were relatively mild and short-lived. As soon as America needed Pakistan's help after the Sept. 11, 2001, attacks, the country was transformed from nuclear outlaw to "major non-NATO ally."

Nicholas Eberstadt, a North Korea scholar at the American Enterprise Institute, wonders about this: "You have to think the Iranians are watching how we handle the North Koreans in the next few months. If you won't do anything with a big cheater, what are the middle and future cheaters to think?" The list could include Syria, Saudi Arabia or Egypt, Taiwan or Brazil, even Indonesia and Sudan.

What's been missing in Washington in the last few months - to say nothing of capitals in Asia and Europe - is clear language about what the world is ready to do if Iran and North Korea follow the path of India and Pakistan. Sure, during his first term, Mr. Bush said repeatedly that he would not "tolerate" either country possessing a nuclear weapon. But no one in the White House will say exactly what that means.

In Iran's case, there may be some time to figure it out: Adm. Lowell E. Jacoby, director of the Defense Intelligence Agency, told the Senate last week that Iran is unlikely to be able to make a weapon until "early in the next decade." (Other intelligence officials think he may be too optimistic.)

In North Korea's case, red lines may be what Kim Jong Il sees in his rear-view mirror. His country declared publicly this month what it whispered long ago to American negotiators: It already has the weapons Mr. Bush will not tolerate.

Maybe Mr. Kim is bluffing. Maybe he isn't. So far there hasn't been a test, and as the North Koreans know, American spy satellites are in the sky over the country, looking for evidence that one may be imminent.

But there is no doubt that the North has stepped up to every line the world has drawn in the sand, and then inched over it, waiting for the reaction. This time, the reaction has been pretty muted. The North's angry declaration 10 days ago led the United States and its neighbors to warn that it must return to six-nation talks about disarmament that have gone nowhere in 18 months, and haven't convened since last June. But punishment? The Chinese and South Koreans say it would only incite the North Koreans to some other provocation.

Even President Bush, the author of the most muscular national security strategy in memory, has sounded remarkably placid. Asked about North Korea on Thursday, he talked about talks.

"It's counterproductive to draw a red line for North Korea because they will only view it as a challenge," one senior administration official explained recently.

However true, that has left an enormous ambiguity at the core of American policy.

"If, right now, the cases we worry about the most are Iran and North Korea, and if they are seen as compelling dangers to the international system, what is it we are prepared to do to forestall them from going nuclear?" asked Jonathan Pollack, a professor at the Naval War College who writes about the game theory of nuclear behavior.

"I don't know the answer to that question - and I don't think the administration knows the answer to that, either." But he notes that North Korea is a special case. "If it has nuclear weapons right now," he said, "it would be the first time an avowed adversary of the United States took that step since China," 40 years ago.

Perhaps one reason no one wants to draw sharp lines is that, while North Korea and Iran pose a threat, it may not be a direct one. No one doubted that the Soviets, and eventually the Chinese, could land a missile on a major American city. Iran clearly cannot, and North Korea's long-range missile has never been tested. In short, the threat to America is more indirect. Iran can threaten Israel, North Korea can menace Japan, and either could cut a black-market deal with terrorists. (That was why the administration was so sensitive about new evidence that North Korea sold raw nuclear fuel to Libya as recently as 2003.)

But no one understands how to play ) this new game better than North Korea. "They've been very, very smart about how they have gone about this," a senior administration official said the other day. "Bit by bit. Just hoping the world will accept this as the new reality."

It may be working. The other day Pat Roberts, the Republican head of the Senate Intelligence Committee, didn't sound much like a hardliner. "They have whatever capability they have, and they insist that they are going to continue with that, and I think that's the way it is," he told Reuters.

This is exactly what a veteran of the cold war, Brent Scowcroft, who trained many on the Bush team, warned about in 1994. He published an essay urging the Clinton administration to draw a red line: Tell the North Koreans that if they move to turn their stockpile of spent nuclear fuel into bomb-grade plutonium, we will bomb their reprocessing facility. It would be risky, he and his co-author, Arnold Kanter, argued, but he doubted that the North would risk all-out war and the toppling of its regime. It was the only way, he argued, "to prevent a bad problem from becoming worse."

A decade later, facing a different president, the North Koreans did exactly what Mr. Scowcroft warned couldn't be allowed: They kicked out international inspectors and flipped on the plutonium machine. But Mr. Bush, tied up in Iraq, drew no red line. By then, aides say, intelligence showed that the North apparently had a second nuclear program under way, involving uranium. "What good would it have done to stop one program, and not the other," one aide asks.

Conservatives are clearly unhappy that a White House that talked in such clear terms about what Saddam Hussein must do has sounded so unclear on North Korea and Iran. Mr. Eberstadt wrote last week: "Each new round of North Korean nuclear provocations has generated clear-cut benefits for the North Korean state, rather than incontrovertible costs. It will be very unpleasant and very expensive to un-teach Pyongyang the lessons of the past two and a half years."

At the very least, drawing those lines now would be a lot more complicated. In North Korea's case, President Bush has said the problem has to be handled in concert with the North's neighbors - China, South Korea, Russia and Japan. That's the kind of multilateral approach the world has urged, but it also means getting a unanimous position from countries with very different interests. So far, that has been impossible.

Likewise, the Europeans, who are taking the lead with Iran, are not interested in red lines that would choke off trade, especially oil. So they have studiously avoided the question of how far they are willing to let the Iranians go in developing a civilian nuclear capability that could be converted, in a matter of months, to military use.

So the cold war really is over, but its rules have yet to be rewritten. Some want to start doing that this year, when the Nuclear Nonproliferation Treaty is up for review. But for now, a world free of red lines is a world in which any state that has long hankered for a bomb sees an opening to get one - and to hope that by the time new rules are written, it will be too late to turn back the clock.