The United States had planned for a clear-cut victory in Iraq. But while Iraq's forces were quickly defeated, a shadowy insurgency has sprung up in their place.

Lessons learned from past U.S experience in counterinsurgency had laid dormant for many years. Before the invasion of Iraq, Marine Corps Colonel Thomas Hammes had a hard time finding out anything about the U.S. doctrine for counterinsurgency warfare.

"I actually went to Special Forces [headquarters] at Fort Bragg and asked to talk to the guys who dealt with counterinsurgency doctrine. There was one gentleman there about to retire," he said. "He gave me what he thought was the last existing [counterinsurgency] manual and said, 'frankly, nobody's interested, you might as well have this.' And we never shifted from that."

But the military is intensely interested now as it confronts the insurgency in Iraq that has taken more U.S. lives than the fight to defeat Iraq's regular forces. And a new, rewritten version of the army's counterinsurgency manual was issued in October of last year - 18 months after the U.S. attacked Iraq.

The new manual, labeled "interim," says that after the end of the U.S. role in Vietnam, counterinsurgency efforts were limited to special forces. It says that what it terms "the aftermath of instability" after the defeat of Saddam Hussein prompted a review of counterinsurgency doctrine, especially for conventional forces unfamiliar with insurgent warfare. A final version, incorporating new lessons from Iraq and Afghanistan, is to be issued by October 2006.

An insurgency is defined as an armed rebellion against civil government or authority. History is replete with them, and the United States has been involved in attempts to put down many of them, such as in Latin America, Southeast Asia, and now in Iraq.

Speaking at a forum of the American Enterprise Institute, Steven Metz, chairman of the Regional Strategy and Planning Department at the U.S. Army War College's Strategic Studies Institute, says insurgency is as much about psychology as anything else.

"Insurgency is about perceptions, beliefs, expectations, legitimacy, and will. Insurgency, as we all know today, is not won by killing insurgents, not won by seizing territory. But it's won by altering the psychological factors that are most relevant."

Insurgents use guerrilla and terror tactics to make up for their smaller numbers and lesser firepower. In an appearance on VOA's Press Conference USA, Colonel Hammes, currently a senior military fellow at the National Defense University, says that by close-quarters fighting in urban areas, the Iraqi insurgents are able to negate much of the U.S. technological advantage.

"If you're moving around the countryside with a weapon, I can target you and kill you and you'll never know where it came from. But if you're in the urban area, you hide the weapons in the vehicle you're moving around in or you just stay under a roof. The thing that defeats all of our high technology is a wall," he said.

Insurgents are criticized by their opponents for not "playing by the rules." But, Colonel Hammes says, they have no reason to do so. "Rules of war were written by Western nation-states for Western nation-states. One of the things an insurgent does is he doesn't play to your strength. Why would I play by your rules? You wrote the rules," he said.

Mr. Metz says the insurgents also look to Vietnam for inspiration, hoping that by bogging down U.S. forces for a long period, U.S. public opinion against the war will grow.

"So it remains to be seen today whether the U.S. will have the belly for a conflict that may last one or several decades. To paraphrase it and put it in stark terms, it's not clear that time is on our side in Iraq, maybe Afghanistan, maybe even in the global [antiterrorist] counterinsurgency."

Mr. Metz says there is rarely a clear and unambiguous victory in counterinsurgency warfare, and the conflict is usually resolved by negotiation. But, he adds, negotiation becomes politically difficult once you have slapped the label of terrorist on the insurgents.

"The best that can normally be attained is a partial victory, a negotiated settlement with the less extreme part of the insurgents. Because we have cast the insurgencies we face as terrorism or terrorists, that makes accommodating part of the insurgents or fracturing the insurgency very, very difficult. It takes a strategy that has very often worked in the past, of accommodating yourself to part of the insurgents, simply off the table because you don't negotiate, you don't bring terrorists into the government," he said.

The U.S. doctrine is to train the foreign government or administration's forces to gradually take over its own defense against insurgents. This approach, first dubbed "Vietnamization" when it was tried during U.S. involvement in Vietnam, is being tried in Afghanistan and Iraq. Experts say the new forces don't have to be a world-class army. They just have to be better than the insurgents.