Guerilla warfare is the most underrated and the most successfulform of warfare in human history," says Ivan Eland, a specialist on national security at the Independent Institute in Oakland, Calif. "It is a defensive type of war against a foreign invader. If the guerrillas don't lose, they win. The objective is to wait out your opponent until he goes home."

From the Filipino insurrection during the Spanish-American War to Vietnam to El Salvador, American troops have had plenty of experience in fighting home-grown enemies that look nothing like a conventional army. As have France in Algeria, Britain in Malaysia and Northern Ireland, Israel in the occupied territories.

Though "counterinsurgency" calls up memories of Vietnam, there may be as many differences as similarities.

Different from Vietnam

Iraqi insurgents have no means of deploying battalion-size forces, as North Vietnam and the Viet Cong did with help from the former Soviet Union. Iraq won't become a proxy conflict between superpowers, as the Vietnam War was. There is a heavy criminal dimension to the violence in Iraq, just as there has been in Algeria, Colombia, and Chechnya. And there is unlikely to be a negotiated resolution as long as Iraq is seen as part of the broader war on terrorism.