News of North Korea’s nuclear test should make Americans worry all the more about Iran, the “other” proliferation challenge. The question is, what do we do about it?

The permanent members of the U.N. Security Council have agreed in principle to impose sanctions on Iran, but the details are still up in the air. The prospects are not promising.

It has always been hard to dissuade a country from developing nuclear weapons. The notable successes have been when great powers could influence dependent allies.

That’s how America derailed nuclear programs in Taiwan and South Korea, and curbed whatever urges Japan and West Germany might have had to “go nuclear” — with diplomatic pressure, economic incentives, and military assurances.

Diplomatically isolated states, in contrast, are tougher customers — witness South Africa, for example, and now, if its claims are true, North Korea. These countries are more likely to be accustomed to “going it alone,” and willing to risk using subterfuge. Countries that face potential adversaries with nuclear weapons — as Pakistan did — are also hard to dissuade.

Iran fits the latter mold. It has few close allies. It is surrounded by nuclear states. And it is rich enough not to worry much about sanctions. Moreover, China, France, and Russia all have substantial economic interests in Iran, and any one of them can veto a Security Council resolution. That’s why the United Nations is unlikely to impose meaningful sanctions, and sham sanctions will only make us look weak.

But suppose Iran did agree to curtail its nuclear program. What then?

Today’s machinery for building nuclear weapons is smaller, cheaper, and easier to acquire. The current proliferation technology of choice — gas centrifuges for enriching uranium — can be buried, hidden, and dispersed. Designs for first- and second-generation centrifuges have been around since the 1980s and are readily available, thanks largely to A.Q. Khan, the former head of Pakistan’s nuclear program.

So even if we do wrangle an agreement from Iran, it will be hard to monitor and impossible to verify. Tehran will have the incentive to prepare the groundwork for a sudden breakout, or to develop a concealed parallel program — just as North Korea apparently did after signing the Agreed Framework that was supposed to end its nuclear weapons program in 1994.

Iran’s drive for nuclear weapons is unlikely to disappear. Imagine you were an Iranian leader, surrounded by Israel, Russia, China, Pakistan, and India — all established or (in the case of Israel, assumed) nuclear powers. Would you agree never to develop a nuclear hole card?

This is why, according to most polls, Iran’s nuclear program is popular with almost every sector of Iranian society — including opponents of the current theocracy. If any Iranian leader agreed to nuclear controls, his rivals would likely use the issue to rally support — as the Bharatiya Janata Party did in India in 1998.

What’s more, the same features that make Iranian nuclear facilities so hard to monitor also make them hard to destroy with a military strike. If you can’t see them, you can’t target them, and if they are buried, they are even harder to destroy.

Even worse, a military strike would weaken the elements in Iran who oppose the regime and discredit those who are more sympathetic to America. And Iran could easily retaliate — say, by making even more trouble for American forces in Iraq and stepping up its support for Hezbollah in Lebanon.

We need to focus more on deterring Iran, and we need to start now, even as we continue to oppose the Iranian nuclear program and hinder it however we can. At least three measures would help.

First, make nuclear weapons less attractive to the Iranian public. Iranians missed out on the Cold War experience we had, as we teetered on the edge of nuclear war. Currently Iranians are only being presented the security and prestige benefits of a nuclear program.They need to become more familiar with the costs and risks.

Iranian citizens must understand that the use of nuclear weapons — at least against a nuclear-armed opponent — inevitably elicits retaliation. Using a surrogate — like a terrorist organization — to deliver a bomb is no escape when your leader has said in the past that the most likely targeted country — Israel — “must be wiped off the map.”

Iranian leaders must understand that if they trigger such a nuclear war, they will be held responsible for the destruction of 5,000 years of Persian culture. We can put these messages directly to the Iranian people. Iran is not a sealed society.

Second, we should base American policy on bounds that we can verify and other countries will support. We may not be able to verify whether the Iranians have built a nuclear weapon, but we can detect whether they have set one off, and even France, China, and Russia would agree that should never happen.

Setting the limit at testing would also help defuse criticisms from our Muslim allies why we tolerate Israel’s nuclear program. There is a world of difference — political, military, diplomatic — between a program everyone assumes and a program everyone has to acknowledge.

We can also likely detect the transfer of Iranian nuclear weapons technology. No one wants that to happen, either, and other nations would join us in countermeasures.

Third, we should stop enhancing the political and military value of an Iranian bomb. Paradoxically, the more vocal we are in our objections to the Iranian nuclear program, the more we make it an effective bargaining chip. The cost of eliminating it rises, in our dealings with Iran and with our partners.

This is the time for quite, serious diplomacy combined with developing real defensive and retaliatory capabilities that are apparent to all. This would put questions in the minds of Iranian military leaders who would plan a strike, and undercut any official who might actually propose to use a nuclear weapon.

This is not an ideal solution. But the potential of an Iranian nuclear weapons program will not go away. So we need to manage it, cripple it, and devalue it. As in so many cases today, we must prepare and pace ourselves for a threat that will be with us for a very long time.

Mr. Berkowitz is a research fellow at the Hoover Institution at Stanford University.