By Richard N. Haass

NEW YORK ― History rarely unfolds smoothly or evenly. Instead, it tends to be punctuated by major developments ― battles, assassinations, breakthroughs ― that have consequences that are felt for years.

Thirty-one years after the revolution that ousted the Shah and brought Islamic rule to Iran, we are at one of those turning points.

To be sure, we do not know the degree, direction, or pace of change. What we do know, however, is that what happens in Iran will materially affect not just that country but the entire Middle East and beyond.

One future for Iran would be mostly an extension of what already exists, i.e., an Iran run by conservative clerics and an aggressive Revolutionary Guard, with the latter increasingly enjoying the upper hand.

The Iranian regime would continue to brutally repress its domestic opponents, meddle in Iraq and Afghanistan, arm and fund Hezbollah and Hamas, and, most important, develop the ability to construct one or more nuclear weapons and the means to deliver them.

The emergence of such a future would present the world with a stark choice: either acquiesce to an Iran that possesses or could quickly assemble a nuclear device, or launch a preventive military attack designed to destroy much of the Iranian nuclear program.

Iran's emergence as a nuclear-weapons state would almost certainly tempt several of the main Sunni Muslim countries (Turkey, Egypt, and Saudi Arabia all come to mind) to embark on a crash program to acquire or develop nuclear arms of their own.

A Middle East comprising several nuclear weapons states is a recipe for catastrophe. An armed attack by the United States, Israel, or both on Iran's nuclear facilities is another possibility.

One downside of such a prospect that Iran would likely retaliate against U.S. interests and personnel in Iraq and Afghanistan, and, using Hamas and Hezbollah, against Israel and others.

Iran could also interfere with oil traffic, leading to a spike in prices and delivering a further blow to American and global economic recovery.

Moreover, while a preventive strike would delay Iran's nuclear efforts, it would not stop the regime from rebuilding, and it might also create conditions that cause problems for the regime's domestic opponents.

But, despite these potential drawbacks, an armed attack on Iran's nuclear facilities will and should remain a distinct possibility given the enormous strategic costs of a nuclear-armed Iran.

It is in part to avoid the difficult choice of either living with a nuclear-armed Iran or attacking it that the five permanent members of the United Nations Security Council and Germany have pursued negotiations to limit Iran's nuclear program and place it under international supervision.

Russia and China, which claim to oppose the emergence of a nuclear-armed Iran, are now being pressed to support new, tough sanctions to increase the odds it does not happen.

But if history is a guide, even strong sanctions may not be enough to persuade Iran's rulers to negotiate constructively and accept meaningful constraints on their nuclear activities.

These considerations raise the prospect of trying to bring about an alternative future: an Iran with a political leadership that is more moderate at home and abroad, and that forgoes developing a nuclear weapon or anything close to it.

In addition to providing a better life for Iran's 70 million people, political change there would weaken both Hamas and Hezbollah, thereby strengthening the relative position of moderates in the West Bank and Gaza and much improving the prospects for peace between Israel and the Palestinians.

A more moderate Iran would also cause Turkey to reconsider its recent shift away from the West and lead Syria to rethink its foreign-policy orientation, which would create a real opportunity for an Israeli peace deal with Damascus.

Iraq's prospects for emerging as a successful country at peace with itself and its neighbors also would be much improved.

It is rare in history that such widely different but plausible paths stem from a common point. It is not difficult, however, to determine which one is preferable.

This is why additional measures are called for to improve the prospects for political change that brings about an Iranian government prepared to live in peace with its own people and its neighbors.

Such measures include assisting the Green Movement so that it can maintain access to the Internet, introducing additional sanctions aimed at the Revolutionary Guard, and publicly supporting the political and legal rights of the Iranian people.

Some governments and individuals are likely to resist these suggestions, believing that such intervention constitutes an unwarranted intrusion into Iran's sovereignty.

But in today's global world, what happens in Iran is more than Iran's affair. Iran's government has a right to nuclear power to generate electricity, but not to a nuclear weapon.

It also has obligations to its neighbors, to the world community (not to support terrorism, for example), and to its citizens. The world should not sit idly by as Iran's regime fails to meet these obligations.

Richard N. Haass is president of the Council on Foreign Relations and author of ``War of Necessity, War of Choice: A Memoir of Two Iraq Wars." For more stories, visit Project Syndicate (www.project-syndicate.org).