But in an intelligence void that even U.S. officials acknowledge, little is known about the insurgents' numbers and whether or how they work together. More important, experts are unsure whether the attacks mark the beginning of a national resistance movement, with popular support, that could present a far deadlier challenge to American forces.
The toll from this new enemy is obvious in 136 American combat deaths since May 1, the withdrawal of United Nations staff and aid workers, and the choking off of desperately needed investment. Yesterday, Spain announced it was withdrawing much of its diplomatic staff from Iraq for security reasons, the third nation in the U.S.-led coalition to do so.
Although violence has occurred in all parts of the country, U.S. officials say deep-seated resistance to the presence of Americans is limited to a few areas, such as the Sunni Triangle west and north of Baghdad. But American knowledge is limited.
Information about the insurgents is poor in part because of a lack of security and repeated threat of attacks on Americans in Iraq, making it difficult for American agents to get out into the field and cultivate reliable sources, officials say.
This is "a classic occupation problem," said Daniel Byman, a professor at the Georgetown University School of Foreign Service and a Middle East specialist. "Intelligence is always weak at the beginning, yet there is strong pressure to act."
"This is not a capability you build up overnight. It's been allowed to atrophy over a couple of decades," said Andrew Krepinevich, executive director of the Center for Strategic and Budgetary Assessments.
"They're wholly unprepared for this type of activity," a Senate official who follows Iraq closely said of the CIA and other U.S. spy agencies. "They don't know how to do this - infiltrate an entire country, figure out who's running what, who's shooting and who the instigators are."
Iraqi violence is growing increasingly complex, with a wide range of targets, weapons and locations. The insurgents' weapons include suicide bombs, small arms, rocket-propelled grenades, roadside bombs, car bombs, mortars and surface-to-air missiles. In addition to the Sunni Triangle, the attacks have occurred at the American compound in Baghdad, in the Kurdish-dominated north and in the holy Shia cities, Najaf and Karbala.
The violence ranges from gunbattles to the carefully planned bombing of the U.N. headquarters in Baghdad on Aug. 19, which experts say may have drawn on inside knowledge of the workings of the compound.
U.S. officials believe the perpetrators include an uncertain number of Saddam Hussein's Baath loyalists; a "couple of hundred" members of the Kurdish Islamist group Ansar al-Islam; several hundred foreign fighters, including Islamic extremists and "rebels looking for a cause"; and some members of al-Qaida, although officials aren't saying how many or what level of seniority.
"What isn't clear is who is responsible for which attacks," said a U.S. official who spoke on the condition that he not be named. "They may in some instances cooperate in a limited way on logistics," he added. A U.S. military officer in Iraq said foreign forces "clearly are" working with the former Baathists in some cases.
Although Hussein has put out audiotapes championing attacks on the occupation forces, the prevailing view in the U.S. intelligence community is that he is not personally playing a significant role in those attacks.
The U.S. official said there are "pockets of resistance" where "some people are sympathetic to the remnants" of Hussein's regime but insisted that "the majority of the people are supportive of the United States being there and are glad the regime is gone."
Outside the government, close observers see a potential for a national resistance but are split on whether it is developing.
"I really don't see that it's on the verge of becoming national. It's fairly localized," said Phebe Marr, a historian of Iraq. "One of the danger signs to watch for would be cooperation between militant Sunnis and militant Shia."