Islamic Revolutionary Guards Corps

The designations put into play unilateral sanctions intended to impede the Guard and those who do business with it. This was the first time that the United States has taken such steps against the armed forces of any sovereign government.

In the same year, oil revenue helped shore up the Ahmadinejad government by enriching a new ruling class made up of Revolutionary Guard members and members and alumni of the Basij militia who have their hand in nearly every aspect of the economy and the government.

But in June 2009 Mr. Ahmadinejad's legitimacy and Mr. Khamenei's leadership were challenged by Iranians angry over the results of the presidential election. Tens of thousands have flooded the streets of Tehran and elsewhere in demonstrations.

The crisis, the gravest since the Islamic Revolution in 1979, erupted after Iran's Interior Ministry declared that the moderate Mir Hussein Moussavi was defeated by Mr. Ahmadinejad by 63 percent to 34 percent. The demonstrators, who represent a cross section of society and part of the clerical establishment, have called the official results a fraud, demanding a full recount if not a new election.

A few days before the vote, Yadollah Javani, the head of the Revolutionary Guard political office, had said if Mr. Moussavi had a velvet revolution in mind, he would see it "quashed before it is born."

With the nation's ruling class apparently divided by the electoral results, the hard-liners in charge sought to portray the unrest as the work of outsiders. The Guard said it had taken action against "deviant news sites" financed by American and Canadian companies.

On June 22, the Guards issued an ominous warning on their Web site saying that protesters would face "revolutionary confrontation." "The Guards will firmly confront in a revolutionary manner rioters and all those who violate the law," the notice said. Shortly afterward, a group of as many as a thousand demonstrators at Haft-e-tir Square in central Tehran was quickly overwhelmed by baton-wielding riot police and tear gas.

The following day, the Guardian Council, the nation's most powerful oversight panel, refused to nullify the election just a day after it announced that the number of votes in 50 cities exceeded the number of eligible voters there by three million.