Dr. Steven Metz, of the Army War College Strategic Studies Institute, said the insurgency could still be sorting itself out. Yet, he said, "It really is significant that even two years in there hasn't been anything like any kind of political ideology or political spokesman or political wing emerging. It really is a nihilistic insurgency."

He warned that this hydra-headed quality could make the insurgents hard to crush, even as the lack of unity makes it unlikely they will rule Iraq. "It makes it harder to eradicate the insurgency, but it also makes it more difficult for insurgents to gain their ultimate objective — if that is to control the country," he said.

Varying causes

That no one knows if that is the objective is, by historical standards, one of several remarkable, perplexing features of this fight.

A clear cause — one with broad support — is usually taken for granted by experts as a prerequisite for successful insurgency.

But insurgents in Iraq appear to be fighting for varying causes: Baath Party members are fighting for some sort of restoration of the old regime; Sunni Muslims are presumably fighting to prevent domination by the Shiite majority; nationalists are fighting to drive out the Americans; and foreign fighters want to turn Iraq into a battlefield of a global religious struggle. Some men are said to fight for money; organized crime may play a role.

This incoherence is something new. "If you look at 20th-century insurgencies, they all tend to be fairly coherent in terms of their ideology," Metz said. "Most of the serious insurgencies, you could sit down and say, 'Here's what they want.' "

In Iraq, insurgent groups appear to share a common immediate goal of ridding Iraq of an American presence, a goal that may find sympathy among Iraqis angry about poor electricity and water service and high unemployment.

Average Iraqis may distinguish among the groups within the insurgency and their tactics. Still, the insurgents haven't publicly proposed a governmental alternative, and their anti-American message has been muddied by their attacks on civilians and by the election of an Iraqi government that has not asked the Americans to leave.

Battling stability

If the insurgency is trying to overthrow this regime, it is contending with a formidable obstacle that successful rebels of the 20th century generally did not face: A democratically elected government. One of the last century's most celebrated theorists and practitioners of revolution, Che Guevara, called that obstacle insurmountable.

"Where a government has come to power through some form of popular vote, fraudulent or not, and maintains at least an appearance of constitutional legality," he wrote, "the guerrilla outbreak cannot be promoted, since the possibilities of peaceful struggle have not yet been exhausted."