Getting word that a loved one has been killed or wounded in wartime is devastating. Finding out that the casualty was caused by comrades can be even worse.

Such "friendly fire" is at issue in a military proceeding that began Monday in Louisiana to determine whether two American pilots should be court martialed for accidentally killing four Canadian troops and injuring eight in a bomb attack near Kandahar, Afghanistan, last April.

As the US prepares for war in Iraq, the issue of friendly fire is much on the minds of military leaders and defense experts - particularly since such incidents, as a portion of overall casualties, are growing.

Behind increase, a changing military

There's a confluence of reasons behind the trend, which became obvious during the Gulf War (in which friendly fire accounted for 24 percent of US combat deaths - much higher than in previous wars), continued in Afghanistan, and may plague any war in Iraq.

• The United States is dealing with enemies and situations where casualties due to hostile fire are diminishing. American forces are in a position of overwhelming superiority, giving enemies less opportunity to do damage. So friendly fire incidents and other war-zone accidents - which can be much harder to deal with psychologically and politically - are becoming a greater fraction of the whole.

Before the Gulf War, the Pentagon estimated that a battle to oust Iraqi troops from Kuwait would involve more than 600,000 US troops and result in as many as 20,000 casualties (deaths and injuries). Instead, the ground war was over in 100 hours, and the US lost just 147 troops during the hostilities.

But among those, 35 were killed by their own forces.

"To put it simply, we are doing all the meaningful shooting," says Daniel Goure, defense analyst with the Lexington Institute in Arlington, Va. "When the enemy can't kill anything, a single friendly casualty looms large." Dr. Goure also points to an increase in night combat. "Although we can see at night," he says, "we can't always tell who is who."

British and other allied soldiers were lost to friendly fire as well, and thousands more American troops later succumbed to "Gulf War syndrome" - the suspected consequence of US actions such as destroying Iraqi chemical stocks and firing cannon rounds made of depleted uranium, exposing friendly troops to low-level radiation.

This record contrasts sharply with the friendly-fire record from World War II through Vietnam, when such casualties amounted to less than 3 percent of the total. In the Gulf War, more than 17 percent of American casualties were due to friendly fire. This was particularly true in the brief ground war - and especially in the intense tank battles, where more than three-quarters of all damage to US armored vehicles was the result of fratricide.

• Modern war seldom involves "front lines" any more. Instead, fights take place in a violent, confusing swirl of friends and foes, sometimes directed from afar with imperfect communications systems, and often in settings that include opponents indistinguishable from civilians.

"The flaw here is our lack of intimate understanding of the human terrain," says Larry Seaquist, retired Navy warship commander and Pentagon strategist. "While the American military is touting their 'total situational awareness' conferred by advanced sensors, we still have only a very hazy understanding of the human realities."

Captain Seaquist points to the attack on the Chinese embassy during NATO's Kosovo-Serb war and to a series of friendly fire incidents in Afghanistan.

"Expect a lot more of this in Iraq," he says.

• Perversely, advances in technology can increase the danger of friendly fire. For instance, with lasers and Global Positioning System (GPS) devices, combatants are apt to feel more confident that they can come close to their own troops.

One example: Shortly after US troops went to war in Afghanistan, a 2,000-pound bomb wounded a number of US and allied soldiers fighting to gain control of a prison compound near Mazar-e Sharif. The accident was caused when a soldier changed the batteries in his GPS unit and neglected to reset the coordinates, which meant that the bomb (which came from a US aircraft) was directed to his position rather than that of the nearby enemy.

"There are more beyond-visual-range engagements, since we no longer wait to see the whites of their eyes before we fire at the enemy," says John Pike, director of GlobalSecurity.org in Washington.

"So there are more chances for mistaken identity."

In addition, combat jets are becoming so high-tech and complicated that it increases the danger that the pilot will have a hard time keeping up with everything - both inside and outside the cockpit.

One Gulf War veteran recalls that as one of the first commanders of a Navy air wing featuring single-seat F-18 Hornet attack aircraft, he argued for making the plane a two-seater so that there would be another set of eyes to keep track of everything. He was voted down by his fellow aviators who thought they could handle the aircraft by themselves. (It was single-seat F-16s that bombed the Canadians last April.)

At the same time, say some observers, the US military has not worked as hard as it might have to design devices able to distinguish friend from foe on the ground.

"The military services have sometimes made it a higher priority to use high-tech for lethal fire than for making it possible for US troops to talk to and recognize each other, particularly between different services," says defense expert Michael O'Hanlon of the Brookings Institution. "That command-control-communications part of the effort needs to catch up with the lethal-fire part."

• Contemporary soldiers are much more likely than their predecessors to actually fire their weapons. In World War II, fewer than half of all riflemen ever fired at an enemy, according to Army studies - and military historian S. L. A. Marshall puts that figure at less than 25 percent. This was due to fear and lack of sufficient training, but also because many soldiers thought it was wrong to kill - even in wartime - according to other studies.

As a result, the Defense Department changed its training to teach soldiers to shoot reflexively (rather than reflectively) by, among other things, using man-shaped pop-up targets instead of bullseyes.

Such training "maximizes soldiers' lethality, but it does so by bypassing their moral autonomy," writes Maj. Peter Kilner (USA) in a recent edition of Military Review, a publication of the Command and General Staff College at Fort Leavenworth, Kansas. "Soldiers are conditioned to act without considering the moral repercussions of their actions; they are enabled to kill without making the conscious decision to do so."

The result? Firing rates rose to 55 percent in Korea and 90 percent in Vietnam. With additional bullets-per-soldier flying around, the risk of friendly-fire grows.

It's this combination of increasingly lethal firepower, fluid battlefields, complex communications, and the notorious "fog of war" that can add up to friendly fire losses.

"Frequently the cause is an interaction of individual error and the operating environment," says Marcus Corbin, a senior analyst with the Center for Defense Information in Washington.

"Is aggressive action being informally pushed or downplayed?" Mr. Corbin asks. "Is the equipment too complex, or does it give too much information to process effectively in the stress of combat?"

Among US allies, a growing concern

Those who may fight with American forces in Iraq are concerned about friendly fire as well.

Former British Gulf War commander Andrew Larpent recently accused the Ministry of Defense of "serious negligence" in failing to produce an "identification friend or foe" system to prevent such accidental casualties. Lt. Colonel Larpent commanded a unit in the Gulf War that saw nine soldiers killed and 12 seriously wounded when they were mistakenly attacked by a US Air Force A-10 Thunderbolt tank-buster aircraft.

In a letter to the Daily Telegraph last week, Larpent wrote: "Our chiefs of staff and politicians should consider very carefully ... how they will answer to the nation if yet more British soldiers become casualties in similar circumstances."

It's a warning heard by American officials as well.