In seconds, one Briton was dead and four were wounded. Survivors of Friday's attack characterized the American A-10 pilot as a trigger-happy cowboy with no regard for human life. British and U.S. authorities are investigating the incident.
Gen. Richard Myers, chairman of the U.S. Joint Chiefs of Staff, apologized for the mistake on British Broadcasting Corp. television.
"It's the absolute saddest tragedy that any of us can experience," Myers said Sunday. "There is simply no excuse for that."
Yet such events are very difficult, if not impossible, to avoid on a chaotic battlefield crowded with both friendly and enemy units, military experts said. Those conditions are growing more frequent in Iraq as U.S. forces bear down on Republican Guard units defending Baghdad.
Going into the Iraq invasion, U.S. military planners knew friendly fire could be a major source of casualties. During the Gulf War in 1991, 36 of the 148 American dead were killed by their own comrades.
This time around, friendly fire has caused five of 27 British deaths. Dozens of U.S. troops have been injured by their own forces, and the military is still investigating a potential case of fratricide in the combat deaths of nine Marines near Nasiriyah on March 23.
"I wouldn't expect a zero number in any conflict. Accidents have a tendency to happen," said Patrick Garrett, a military analyst at Globalsecurity.org, an Arlington, Va., defense think-tank.
Civilians may find it difficult to understand how the most technologically sophisticated military in the world can't prevent one of its units from attacking another. But in many ways it's the very precision and lethality of the most advanced weapons that makes them so dangerous to friendly personnel.
"These weapons are so accurate that if you target the wrong place you're probably going to kill the people you've targeted," said Mark Burgess, a research analyst at the Center for Defense Information, a Washington, D.C., think-tank.
Since the Gulf War, several steps have been taken to improve "situational awareness," the military's term for knowing where you are in relation to other units on the battlefield.
The Army has a system that sends a signal from each vehicle to a communications satellite. The satellite beams each unit's location back down to a central computer, which plots friendly vehicles on a screen in blue. Enemy vehicles spotted by field units can be plotted on the screen in red. The whole image is constantly updated and sent back out to individual units for troops in the field to see.
"It's a tremendous tool at removing the fog of war," said Thomas Plavcan, the Army's deputy project manager for the system.
Unfortunately, the only U.S. unit fully equipped with the high-tech system is the 4th Infantry Division, a force that has yet to go into the field.
Another system, not yet ready for the battlefield, would employ an "identify-friend-or-foe," or IFF, system like the ones airplanes use to keep themselves from being shot down. Each vehicle would send out a radio signal at a specific frequency, effectively telling other friendly units in the area, "Don't shoot." The signal would be encrypted to prevent enemies from mimicking the signal to disguise themselves as friendlies.
But such systems don't always work. Early in the war, a Patriot missile battery shot down a British Tornado GR4 aircraft. Though it is not yet known what led to the accident, it could have been caused by a failure of the airplane's IFF beacon or the Patriot radar's ability to detect and identify it.
For the time being, coalition ground forces in Iraq depend mostly on various infrared "tags" affixed to vehicles that show up when viewed through night-vision goggles.
But such measures work only when they are used.
British soldiers from the Blues and Royals regiment say they had an infrared tag that the A-10 pilot who fired on them could have identified if he'd taken the time to check.
It's not yet clear if that is why the A-10 pilot failed to identify his targets as allied vehicles. But in the heat of battle, Myers said, such accidents can occur.
"I suppose in the middle of war, which is by nature a very chaotic event, at certain times on the battlefield those things happen," he said. "But I don't ever accept that they're inevitable, and I don't think we should ever stop trying to find means to prevent that from happening."
By Matt Crenson