THE State Department will reportedly soon announce the opening of an Interests Office in Tehran. It may prove to be a major mistake.

For starters, the Interests Section exists now, operating as part of the Swiss Embassy. The move would simply upgrade it with the appointment of at least two senior US diplomats.

Secretary of State Condoleezza Rice says this would enable the United States to gain a more direct understanding of what's going on inside Iran - and, if the time comes, establish more direct communication with the Islamic Republic's leadership. Advocates of accommodation with Tehran go further, claiming the move would signal US acceptance of the regime, thus allaying the mullahs' fear of regime change.

Where to begin? Perhaps with November 1979 - when a huge US diplomatic presence in Tehran proved no guarantee of the regime's good behavior.

President Jimmy Carter had handwritten a personal letter to Ruhollah Khomeini, praising the ayatollah as a man of God and proposing a strategic alliance. National Security Adviser Zbingniew Brzezinski had flown to Algiers to meet Khomeini's prime minister, Mehdi Bazargan, with promises of close ties and supplies of US arms.

None of that prevented the raid on the US embassy that left 65 US diplomats seized as hostages - with 52 remaining in captivity for 444 days. America wasn't dubbed "The Great Satan" for a lack of diplomats in Tehran - but because of its diplomatic presence.

Next: How is it wise to try to remove fear of regime change in Tehran? Once assured that they no longer face the threat of regime change, the mullahs may well decide that they need offer no concessions at all on any issue, least of all that of their nuclear ambitions.

The experience of the last 30 years shows that the only time the mullahs have agreed to compromise on any serious issue is when they face a serious threat of regime change.

* In 1987, the US Navy sunk the Islamic Republic's navy in the Persian Gulf while President Ronald Reagan adopted a war posture against Tehran. Khomeini immediately stopped firing on Kuwaiti and Saudi oil tankers - and finally accepted a cease-fire with Iraq that he had rejected for five years. Faced with the possibility of regime change, he had to (in his own words) "drink a cup of poison" and strike some compromises.

* In 1988, faced with the possibility of a massive Israeli operation against their Hezbollah units in Lebanon, the mullahs agreed to a French-brokered deal under which Hezbollah stopped its terrorist operations in exchange for a promise not to be attacked.

* In 2001, fearing that the US might target them for regime change, the mullahs cooperated with Washington against the Taliban in Afghanistan.

* Last spring, having suffered a decisive defeat in Basra, Iraq, and fearing that the battle might spread into Iranian territory, the Islamic Revolutionary Guard Corps backed down, letting the new Iraqi army win a decisive victory over the so-called Mahdi Army.

President Mahmoud Ahmadinejad would certainly see a US decision to reopen the embassy in Tehran - unconditionally and in all but name - as vindication of his hard-line positions. He would claim that his less-radical predecessors failed to get such concessions from Washington because they'd been "too soft."

Rice might also ponder other unintended consequences of her move. Will the Interests Section attract endless lines of Iranians hoping to get US visas? Ahmadinejad is already forcing dissenters out of the country and (whenever possible) into the United States. In the last three years, he has released hundreds of dissidents on the condition that they leave the country and emigrate to America.

Will the US take in large numbers of political asylum seekers from Iran, while Ahmadinejad gets rid of some of his most vocal critics? Or will the US reject the asylum-seekers, providing a new focus for anti-American feeling?

Certainly, the regime would use the Interests Section to get visas for some of its own members, flooding the US with shady characters. (In fact, a whole regiment of Islamic Republic officials and apologists have already struck roots in the US, lobbying for the regime.)

Rice should also shed any illusions that Interest Section diplomats will gain access to real decision-makers in Tehran: Even the most favored ambassadors have no access to the secretive regime's real sources of power. No Western ambassador has ever met "Supreme Guide" Ali Khamenei. Even Ahmadinejad doesn't receive any ambassadors for anything more than a formal courtesy call.

Finally, many Iranians might see the US move as a sign that Washington is endorsing the regime, warts and all. That would only improve Ahmamdinejad's chances of winning re-election next June.

It would be odd indeed to see the Bush administration, in its last days, helping Ahmadinejad get re-elected.