WASHINGTON — Allegations that U.S. Marines massacred as many as 24 innocent civilians in Haditha provide a dire illustration of the psychological stresses and moral tests that service members face in their fight against Iraq's unrelenting insurgency.

This is a war without front lines against an enemy who wears no uniform. Death can come at any moment, from the blast of a bomb hidden along a road or a mortar round lobbed onto a base. And often, in the angry moments after a comrade's life is ripped away, there is no readily identifiable enemy to confront — only a foreign population in which friend, foe and bystander may seem indistinguishable.

"It's a pressure cooker. It's a 24/7 situation in which you're constantly worried about your safety, about danger," said Matthew Friedman, executive director of the U.S. Department of Veterans Affairs' National Center for Post-Traumatic Stress Disorder.

While no one is arguing that such pressure is an excuse for wartime atrocities, there are clear signs that the mission is taking a psychological toll on U.S. troops. An early study of veterans returning from the Iraq war found one in six showed symptoms of depression, anxiety or post-traumatic stress disorder. A more recent study found that one in three Iraq veterans has sought help from a mental health professional.

Many of the U.S. troops in Iraq are now on their second or third tour of duty in a conflict that has stretched beyond original expectations; some have been forced to remain in the military longer than their original enlistment period.

The Marine unit in Haditha was on its third rotation in Iraq when the incident allegedly occurred Nov. 19. The same month a year earlier, on a previous tour of duty, the unit had been engaged in fierce house-to-house fighting in the battle to retake Fallujah from insurgents.

"The multiple tours just make the pressure worse," said Dr. Arthur Blank, an Army psychiatrist during the Vietnam War and former national director of a Veterans Administration counseling program who now treats Vietnam and Iraq veterans in private practice in Bethesda, Md.

The Pentagon has yet to release its report on the Haditha incident, an investigation begun in February after Time magazine sought comment on its findings that Marines in Haditha had executed non-combatants in homes after a roadside bomb killed a Marine. Among the dead were children and an elderly man who used a wheelchair, according to the magazine's report, which was published in March.

The Defense Department recently warned senior members of Congress that its investigation was pointing toward evidence of a rampage by Marines and a cover-up by superiors. Last week, all U.S. troops in Iraq were ordered to undergo "core values" training covering treatment of non-combatants. On Sunday, Gen. Peter Pace, chairman of the Joint Chiefs of Staff, promised a thorough investigation of the Haditha incident and said it would be wrong to let "the emotions of the day weigh into the process."

The military also has acknowledged it is investigating an incident in Hamandiya in which Marines allegedly took an Iraqi man from his house and executed him by the side of a road.

Speaking to reporters Friday at the Pentagon via a teleconference from Baghdad, Brig. Gen. Donald Campbell, the chief of staff at the U.S. military headquarters in Iraq, said troops "could snap" under the severe pressures of fighting the insurgency.

"It doesn't excuse the acts that have occurred, and we're going to look into them," Campbell said. "But I would say it's stress, fear, isolation and, in some cases, they're just upset. They see their buddies getting blown up on occasion, and they could snap."

Many units in Iraq have faced similar circumstances — more than 2,400 Americans have died in Iraq, often from roadside bombs — but the units have not resorted to atrocities. Still, they have shared many of the same stresses and swirling passions.

"They get attacked and ambushed all the time. And they want to take it out on somebody in some physical way. But you very seldom see the enemy," said Greg Wilcox, a retired Army lieutenant colonel who is a consultant to the military on urban warfare and counterinsurgency operations.

Maj. Peter Kilner, a West Point professor who has done research on combat stress, said the military relies on ground-level leaders, particularly the lieutenants who command platoons and sergeants who run squads, to keep those passions in check.

"The difference between soldiers doing the right thing when they're off on the edge and falling into the abyss is leadership," Kilner said.