Global Policy Forum

By Dexter Filkins

Here is a small sampling of the insurgent groups that have claimed responsibility for attacks on Americans and Iraqis in the last few months: Supporters of the Sunni People. The Men's Faith Brigade. The Islamic Anger. Al Baraa bin Malik Suicide Brigade. The Tawid Lions of Abdullah ibn al Zobeir. While some of them, like the Suicide Brigade, claim an affiliation with Al Qaeda in Mesopotamia and Al Qaeda claims them, others say they have acted alone or under the guidance of another group.

While on Wednesday President Bush promised nothing less than "complete victory" over the Iraqi insurgency, the apparent proliferation of militant groups offers perhaps the best explanation as to why the insurgency has been so hard to destroy. The Bush administration has long maintained, and Mr. Bush reiterated in his speech Wednesday, that the insurgency comprises three elements: disaffected Sunni Arabs, or "rejectionists"; former Hussein government loyalists; and foreign-born terrorists affiliated with Al Qaeda.

Iraqi and American officials in Iraq say the single most important fact about the insurgency is that it consists not of a few groups but of dozens, possibly as many as 100. And it is not, as often depicted, a coherent organization whose members dutifully carry out orders from above but a far-flung collection of smaller groups that often act on their own or come together for a single attack, the officials say. Each is believed to have its own leader and is free to act on its own.

Highly visible groups like Al Qaeda, Ansar al Sunna and the Victorious Army Group appear to act as fronts, the Iraqis and the Americans say, providing money, general direction and expertise to the smaller groups, but often taking responsibility for their attacks by broadcasting them across the globe.

"The leaders usually don't have anything to do with details," said Abdul Kareem al-Eniezi, the Iraqi minister for national security. "Sometimes they will give the smaller groups a target, or a type of target. The groups aren't connected to each other. They are not that organized."

Some experts and officials say there are important exceptions: that Al Qaeda's leaders, for instance, are deeply involved in spectacular suicide bombings, the majority of which are still believed to be carried out by foreigners. They also say some of the smaller groups that claim responsibility for attacks may be largely fictional, made up of ragtag groups of fighters hoping to make themselves seem more formidable and numerous than they really are.

But whatever the appearances, American and Iraqi officials agree on the essential structure of the Iraqi insurgency: it is horizontal as opposed to hierarchical, and ad hoc as opposed to unified. They say this central characteristic, similar to that of terrorist organizations in Europe and Asia, is what is making the Iraqi insurgency so difficult to destroy. Attack any single part of it, and the rest carries on largely untouched. It cannot be decapitated, because the insurgency, for the most part, has no head. Only recently, American and Iraqi experts say, have they begun to grasp the new organizational structure that, among other things, is making the insurgency so difficult to stop.

"There is no center of gravity, no leadership, no hierarchy; they are more a constellation than an organization," said Bruce Hoffman, a terrorism expert at the Rand Corporation. "They have adopted a structure that assures their longevity."

The insurgency's survivability presents perhaps the most difficult long-term challenge for the Iraqi government and American commanders. The primary military goal of groups like Al Qaeda and Ansar al Sunna is not to win but simply not to lose; to hang on until the United States runs out of will and departs. Even killing or capturing the leader of Al Qaeda in Mesopotamia, Abu Musab al-Zarqawi, many Iraqi and American officials say, will not end the rebellion.

In a war as murky as the one in Iraq, details about the workings of the insurgency are fleeting and few. But what is available suggests that the movement is often atomized and fragmented, but no less lethal for being so. A review of the dozens of proclamations made by jihadi groups and posted on Islamist Web sites found more than 100 different groups that either claimed to be operating in Iraq or were being claimed by an umbrella group like Al Qaeda. Most of the Internet postings were located and translated by the SITE Institute, the Washington group that, among other things, tracks insurgent activity on the Web. Of the groups found by SITE, 59 were claimed by Al Qaeda and 36 by Ansar al Sunna. Eight groups claimed to be operating under the direction of the Victorious Army Group, and five groups said they were operating under the 20th of July Revolution Brigade.

The complex nature of the insurgency was illustrated on Oct. 24, when three suicide bombers, one driving a cement mixer full of TNT, staged a coordinated attack on the Palestine and Sheraton Hotels in central Baghdad. The attack was one of the most sophisticated yet, with the first explosion ripping open a breach in the hotels' barriers. That allowed the cement mixer to come within a few yards of the Sheraton before being hung up in barbed wire.

An American solider opened fire on the driver of the truck, and the bomb was apparently detonated by remote control. Twelve people died, and American and Iraqis agreed later that the attack had come very close to bringing both towers down. Within 24 hours, Al Qaeda, in an Internet posting viewed round the world, boasted of its role in attacking the "crusaders and their midgets."

But in the small print of the group's proclamation, Al Qaeda declared that the attack had actually been carried out by three separate groups: the Attack Brigade, the Rockets Brigade and Al Baraa bin Malik Suicide Brigade. The three groups, the Qaeda notice said, had acted in "collaboration," with some fighters conducting surveillance while others provided cover fire.

Rita Katz, the director of SITE, which is now working under a United States government contract to investigate militant groups, said the attack on the Palestine and Sheraton Hotels had probably been planned and directed at the highest levels of Al Qaeda. The leaders may have brought the three "brigades" together to stage the attack, she said, and probably provided expertise as well as the suicide bombers themselves. "This was something that was coordinated at the highest level," she said.

But for most of the attacks, such top-down coordination is uncommon, Ms. Katz and American and Iraqi officials said. Most, they said, are planned and carried out by the local groups, with the leaders of the umbrella groups having little or no knowledge of them. American and Iraqi experts also say there appear to be important distinctions among the umbrella groups. While Islamist groups like Al Qaeda and Ansar al Sunna attack military and civilian targets at will, other organizations, like the Victorious Army Group, which is believed to be associated with followers of Saddam Hussein's government, appear to attack only American or Iraqi solders.

In recent months, some insurgent groups have refined their target goals even further. In July, Al Qaeda said it had formed a group called the Omar Brigade to focus on killing members of Shiite militias like the Badr Brigade. Since then, the Omar Brigade has taken responsibility for dozens of killings.

Some insurgent groups appear to be limited to exclusive geographic areas. The Zi al Nourein Brigade, whose exploits are regularly proclaimed by Ansar al Sunna, appears to operate almost exclusively in Mosul, in northern Iraq. Each week, more such groups announce their presence. "Following Allah's orders to his worshipers, the mujahedeen, to join together and stand in one line against Allah's enemies," a posting on the Internet said July 12, "Al Miqaeda Brigade Groups announced that they are joining Ansar al Sunna."

American and Iraqi officials say they are not always sure that the groups' public claims of responsibility are valid. It is an old trick that guerrilla movements use to exaggerate their size and power. Other experts who track jihadi Web sites say it is possible to authenticate the claim of an attack by a particular group. Most of the claims of responsibility appear on Web sites that tightly control access to their message boards.

The array of insurgent groups has prompted competition among them. On the streets of Ramadi, the violent city west of Baghdad, a leaflet found on the street, signed by a group called the Islamic Army, said that "the growing number of mujahedeen groups, which grew in number when the people realized their value," had caused confusion about which group was speaking for which. The Islamic Army leaflet read like an advertisement offered by a product manager worried about imitators. "We are asking people to reject any statement signed by the Sajeel Battalion of the Islamic Army that does not carry their slogan or seal," the leaflet said.

One question that remains unsettled is the nationalities of suicide bombers. American and Iraqi officials have long said they believe that the majority of suicide attacks are carried out by foreigners. In June, in an apparent answer to that question, Al Qaeda announced the formation of the Ansar Brigade, which it described as an all-Iraqi suicide unit. Since then, the Ansar Brigade has taken responsibility for few such attacks.

One place where the Ansar Brigade did apparently strike was Jordan last month, when suicide bombers struck three hotels in Amman. The police there determined that Iraqis had carried out the attack. In a message posted on the Internet, Al Qaeda announced that the Ansar Brigade, its Iraqi suicide group, had carried out the attack.

Sabrina Tavernise contributed reporting from Ramadi, Iraq, for this article.