What are cause of fratricide?

Answer 1

In the military sphere, fratricide is the accidental killing of friendly forces when the intent was to attack the enemy. Suicide, "fragging" (and similar intentional killing of friendly forces) and accidental deaths of civilians or non-combatant forces are not included in fratricide.

The two major causes of fratricide are mistaken identification of friendly forces as enemy forces, and mistaken (or improperly reported/disseminated) position of friendlies.

In the first, the fratricidal force mistakes the friendly force as an enemy, due to a variety of causes. Poor visibility has historically been the greatest cause here, with the next most common cause being a close visual resemblance between friendly and enemy forces causing misidentification. Malfunctioning IFF (Identify Friend or Foe) and broken communications links are increasingly responsible for this problem in modern militaries.

In the second case, the fratricidal force has incorrect information about the location of friendly forces. This may be due to a breakdown in communications, where the fratricidal force does not receive proper updates as to new movements of friendlies, or it may be the friendlies fault for either moving into an incorrect position or not reporting their new movements up the chain of command. It may also simply be a result of mistaken geolocation on either party, where one or both parties think they are in a different location than they really are. Even in the age of GPS, this is still a common problem. These mistakes in location lead to mistakes in assumptions about which types of forces are in there, leading to fratricide (particularly, from indirect fire units attacking beyond line-of-sight targets).

Answer 2

"Fratricide" means, literally, "killing your brother". I don't think it would be possible to list all the reasons people have done that through the years.