INTERNATIONAL CONCERN over Iran's nuclear program continues to smolder. But from Iran's perspective, Iraq has now become the most dangerous flashpoint of potential conflict with the United States.

On a recent visit, I found Tehran seething over what it sees as a new, "divide and rule" US strategy designed to make Iraq a permanent US protectorate.

Nominally, the United States continues to support the Iran-tilted Shi'ite government of Prime Minister Nouri al-Maliki. But in Iranian eyes, General David Petraeus is fencing in Maliki by building up rival Sunni militias under US control.

Until now, I was told, Iran has been actively helping the United States to stabilize Iraq during the "surge" by reducing its weapons inputs to Shi'ite militias, including the Mahdi Army of Moqtada al-Sadr, who has ordered a cease-fire under Iranian pressure. But the message was clear: Unless Petraeus drastically cuts back the Sunni militias, Tehran will unleash the Shi'ite militias against US forces again and step up help to Maliki's intelligence service, the Ministry of National Security. The United States has created a rival agency under Sunni control, the National Intelligence Service.

The tensions building between the Maliki government and the Bush administration over Iran's role in Iraq were underlined recently when Maliki, with visiting President Mahmoud Ahmadinejad of Iran at his side, said that Iran "has been very helpful in bringing back security and stability to Iraq." Two days later, Lieutenant General Ray Odierno, the retiring deputy commander of US forces in Iraq, criticized Iran for continuing to "train surrogates, fund surrogates, and supply weapons to them."

The burgeoning US-sponsored Sunni militias so far number some 90,000 US-equipped fighters who are each paid $300 a month. This is euphemistically called the "Sunni Awakening." The militias pose a growing challenge to the dominance of Maliki's predominantly Shi'ite army, with its authorized strength of 186,000. Abdul Aziz al-Hakim, the key Shi'ite leader backing Maliki, has repeatedly complained that "weapons should be in the hands of the government only, and the government alone should decide who gets them. The alternative will be perpetual civil war."

Iran's former deputy foreign minister, Mahmoud Vaezi, told me that arming the Sunnis "suggests to us that the US is deliberately seeking to keep them strong enough to undermine al-Maliki and contain our influence. It will be impossible for us to cooperate in stabilizing Iraq if this goes on. If you shift power to the Sunnis, then some Shia groups will say, 'If we can get more power through terrorist tactics, why not?' "

President Bush attempts to justify an indefinite US military occupation of Iraq as a counter to Iranian influence. But the reality is that Iran will have dominant influence in Iraq whether or not a stable government emerges in Baghdad and whether or not US forces remain. History and ethnic arithmetic make this the inescapable legacy of the US invasion.

Shi'ites make up 62 percent of the Iraq population. Yet for five centuries, the Ottoman and British invaders who preceded Saddam Hussein, using classic divide-and-rule tactics, installed successive Sunni minority governments to contain the Shi'ite majority. By destroying the Sunni-dominated Hussein regime, Bush gave the Iraqi Shi'ites an unprecedented opportunity to rule that they are now determined to exploit.

Iran and the United States have a common interest in a stable Iraq. Tehran does not want a breakup of Iraq along ethnic lines that would strengthen the movement for an independent Kurdistan embracing its own restive Kurdish areas. Before cooperating to stabilize Iraq, however, Iran wants assurances that the United States will not use it as a base for covert action and military attacks against the Islamic Republic and will gradually phase out its combat forces.

Cooperation will endure only if Washington lets the Shi'ites enforce the terms for the new ethnic equation in Iraq and, above all, if it recognizes Iran's right by virtue of geography and history to have a bigger say in Iraq's destiny than its other immediate neighbors, not to mention the faraway United States.

Selig S. Harrison is director of the Asia program at the Center for International Policy.