FRESH FROZEN; FRIENDLY FIRE

U.S. soldiers killed eight Afghan policemen Tuesday after mistaking them for Taliban forces. A spokesman for the U.S.-led coalition force says the police fired on them first. How do troops normally avoid friendly fire?

With "combat identification systems." These vary depending on the combat situation. For example, airplanes talk to each other using a system known as Identification Friend or Foe, a call-and-response technology similar to that used by air traffic control. An "interrogator" device on one plane sends a coded signal to a transponder aboard another aircraft. If the plane replies, it is considered friendly. Ground troops use several different systems to separate friends and enemies. One works a lot like IFF, but for ground vehicles instead of planes. Another system, called Blue Force Tracking, takes advantage of satellite communications to map out all the friendly units in a certain area, but doesn't refresh in real time. While these technologies can help to identify the positions of friendly vehicles, they don't provide much information about individual soldiers.

Not all battlefield ID methods are so high-tech. During Operation Desert Storm, ground troops wore infrared beacons known as Darpa lights and Budd lights, which were visible to anyone with night-vision goggles at up to 5 miles. (This method is less useful now that night-vision technology has become more available to enemy forces.) Troops also signal their allegiance using glow sticks or thermal tape arranged in different shapes. During the first Gulf War, for example, coalition forces taped a "V" onto their uniforms.

To avoid deadly mistakes, soldiers are expected to follow rules of engagement detailing the appropriate action for every situation, both offensive and defensive. Each country has its own rules, often tailored to specific operations, as do coalitions like NATO. In 2005, American soldiers received "anti-fratricide training" to reduce the number of attacks on British and coalition forces, while the British were instructed to display the Union Jack prominently on their vehicles and approach American convoys slowly.

Fratricide, also known as amicicide, "friendly fire," or "blue-on-blue" hostility, has always been a problem for modern armies. During the French and Indian War, a ship commanded by Gen. George Washington and another commanded by a British officer fired on each other, each thinking the other was French. Gen. Thomas J. "Stonewall" Jackson died after catching three friendly bullets in the right hand and left arm. During World War II, Korea, and Vietnam, estimates of the rate of American casualties resulting from friendly fire ranged from 2 percent to 16 percent. More recently, the 2003 attack on a British convoy by U.S. aircraft gained international attention after a video featuring the pilots' frustrated dialogue leaked to the press. The death of soldier and former NFL player Pat Tillman in 2004 was also the result of friendly fire.