Fratricide, in a military context, happens when members of one's own forces are hit by fire from the same side, or weapons interfere with one another such that their effect is neutralized. This is often called "friendly fire", although the apocryphal "Murphy's Laws of Combat" mention:
- Friendly fire isn't.
- When the pin is pulled, Mr. Grenade is no longer our friend.
Fratricide is the preferred professional term, as no friendliness is involved; it is something to be prevented. Fratricide is a subset of deconfliction, is the part of mission planning that tries to ensure that all preplanned attacks know the position of friendly forces. One of the drivers of network-centric warfare is giving all units and personnel improved situational awareness, such that a unit that moves to take advantage of a sudden enemy weakness is not incorrectly identified as an enemy force to be engaged.
The employment of friendly weapons and munitions with the intent to kill the enemy or destroy his equipment or facilities; which results in unforeseen and unintentional death or injury to friendly personnel. — U.S. Army Training and Doctrine Command
It is an especially severe problem in coalition warfare, where not all participants are familiar with one another, or may not have fully interoperable communications and navigation. Both in the Second World War and the Vietnam War, 15-20% of US casualties were the result of fratricide. In the 1990 Gulf War, which had an even higher operational tempo and more participants, the rate had increased to 24%. It is by no means limited to the US; "Britain has historically been one of the worst offenders. In 1471, during the War of the Roses, the Lancastrian division fired on its forces by mistake. During World War two, submarine HMS Triton sank fellow Royal Navy submarine HMS Oxley and, in the 1982 Falklands War, HMS Cardiff shot down a friendly Gazelle helicopter."
Fratricide against one's own troops has a variety of causes. In a fast-moving battlefield, perhaps the most common cause is improper identification. It is worth noting that use of identification-friend-or-foe technology preceded common use of the terms "friendly fire" or "fratricide." Positive identification is a major preventive step.
Formally, the United States Department of Defense defines "friendly fire" as
In casualty reporting, a casualty circumstance applicable to persons killed in
action or wounded in action mistakenly or accidentally by friendly forces actively engaged with the enemy, who are directing fire at a hostile force or what is thought to be a hostile force.
— United States Joint Chiefs of Staff
The term was popularized during Vietnam, in the book, by New York Times reporter C.D.B. Bryan, about an incident where families could not find out why their son died. 
In swarming (military) and other forms of operations in which multiple friendly directions strike in constantly changing time and space, real-time communications are key in avoiding fratricide. Even so, without computer assistance to recognize impending fratricide, people may be overloaded with information and make errors in recognizing their own side.
Situational awareness or a common operational picture is the ideal. Such awareness goes beyond direct identification, to include common map coordinates and "safe zones".
Stealthy platforms, such as submarines, are especially prone to fratricide because they tend not to have active identification. In the Second World War, submarines in such lanes were still attacked and destroyed by their own side's anti-submarine warfare forces. In October 1943, the destroyer escort USS Rowell sank the USS Seawolf. A sister escort to Rowell had been torpedoed, Rowell established sonar contact with what her captain assumed to be the enemy that had just torpedoed a friendly ship.
Creeping barrages were introduced in the First World War, in which artillery fired just forward of the projected position of advancing troops.
Identification can be passive or active. The most basic identification is a flag or uniform; there were incidents of fratricide among sailing warships when the flag blew in the wrong direction. Since the enemy can duplicate insignia, a recognition sign may be applied just before combat, such as the upside-down "V" markings on Coalition vehicles during Operation DESERT STORM. More recent passive identification insignia are not visible to the naked eye and thus harder to duplicate, but are highly visible in infrared scanners.
One active identification program involves France, Germany, Great Britain and the United States. Among the technical challenges of active identification systems is that a continuously transmitting one can be used, by the enemy, to detect a target. Modern systems are intended to be activated only by a "shooter", which sends a radio or laser signal to the target just before firing. A target, with the appropriate equipment, will respond with an "I'm a friend. Don't kill me" signal, on a narrow beam to the shooter. 
A special case is when one's own weapon has a failure that causes them to attack the launching platform. In WWII, circular torpedo runs sank USS Tullibee and USS Tang, and other submarines had near-misses. While there has never been an official public explanation of the sinking of the USS Scorpion, most theories suggest the incident was torpedo-related, with a malfunctioning torpedo either exploding onboard, or, after being jettisoned, made a circular run.
Inadvertent attacks on friendly personnel and units
Fluid situations and rapidly moving forces tend to increase the risk of friendly fire, as do communication problems with one's allies. For example, Canadian forces driving through Belgium and Holland during the Second World War were bombed by both American and British aircraft.
Friendly fire can also result from bugs in weapons systems or deficiencies in training and documentation that lead to errors by field personnel. The worst friendly fire incident of the Afghanistan War (2001-) took place at the Battle of Kandahar,  when a US soldier used a GPS device to sight in on a Taliban position and had the battery die before he could transmit the target co-ordinates to the bombers. He replaced the battery and transmitted. Unfortunately, the device defaults back to its own position on power-up. The bombers duly came and hammered the co-ordinates they were given. They hit the headquarters of an Afghan force under future president Hamid Karzai, accompanied by U.S. Special Forces Operational Detachment ODA 574, killing 30 and wounding many more.
Inadvertent interference with one's own weapons effects
It can affect both people and materials; there are a number of nuclear warfare scenarios where the explosions from earlier bombs and warheads interfere with missiles and aircraft that must travel through turbulent air or intense radiation.
- Will Roberts (October 12, 2007), "Friendlier Fire", Army Technology
- US Department of Defense (12 July 2007), Joint Publication 1-02 Department of Defense Dictionary of Military and Associated Terms. Retrieved on 2007-10-01
- C. D. B. Bryan (1978), Friendly Fire, Putnam, ISBN 0525704388
- Orloff, Lars R.H. (September 1999), Analysis of Fratricide in United States Naval Surface and Submarine Forces in the Second World War., U.S. Naval Postgraduate School, ADA374561, p. 32
- Reed, Fred V (May 1998), "Atlantic alliance efforts provide common combat identification", Signal
- "GPS default setting blamed in deadly friendly fire event", Washington Post, April 24, 2001