For the United States, the good news is that rebels lose more often than they win. Chances for stopping an insurgency improve after 10 years, the study shows.

Stopping the violence in Iraq will take years, Pentagon leaders have said. However, there have been few efforts to analyze and quantify insurgencies in order to draw conclusions about Iraq and Afghanistan.

"The violence in Iraq is going to go on a minimum for at least three or four more years and in reality another five plus years," said Christopher Lawrence, director of The Dupuy Institute, which is conducting the study.

The Iraq war is in its fifth year.

The Annandale, Va.-based Dupuy Institute is under a Defense Department contract to study insurgencies to help give commanders more information about what works and what doesn't. The study is due to be completed in September.

The military recently produced a new counterinsurgency manual that establishes doctrine for waging a counterinsurgency.

According to the manual, defeating an insurgency requires:

•An understanding of local society;

•Good intelligence about the enemy;

•Establishing security and a rule of law;

•Establishing a long-term commitment.

The new doctrine points out the limits to using overwhelming firepower, which could anger civilians, and the need to find political solutions to win over the population.

The manual says counterinsurgency is much more complex than other forms of warfare, requiring the coordination of political, military and economic efforts.

As part of the study, the institute built a database of 63 post-World War II insurgencies, including Vietnam, the French in Algeria and the Soviets in Afghanistan.

The United States experience in Vietnam soured the U.S. military on insurgencies, Lawrence said. The prevailing military doctrine after Vietnam emphasized building conventional capabilities to counter the Soviet threat. "The subject (of counterinsurgencies) has not been seriously analyzed by the Army since the 1960s," Lawrence said.

Not all insurgencies are quagmires, the report shows. Insurgents only win in 41% of the conflicts in the database, Lawrence said. The remainder were victories for the counterinsurgents, were inconclusive or are still going on.

One of the most successful counterinsurgencies was the British victory over communist insurgents in Malaysia during the 1950s.

Col. Timothy Reese, director of the Combat Studies Institute at Fort Leavenworth, Kan., cautions against reading too much into it.

Each conflict is unique, and the differences are as important as the similarities, Reese said.

"War cannot be reduced to a formula," Reese said. "War is an art as much as it is a science."