Monster infighting

In Doom, many of the enemy monsters can be tricked into fighting each other, or infighting. This can usually be accomplished by moving so that the player and two monsters are colinear. Since a monster does not have the intelligence to cease fire in the event that a fellow monster blocks his line of sight, said fellow monster will be subject to friendly fire. Monster infighting, as it is called, will soon follow.

Monsters of the same type or species, however, are immune to each other's projectiles (with the exception of firearms). This means that causing two identical monsters to fight each other is very difficult due to the fact that the above procedure will be ineffective. To cause homogeneous monsters to fight each other, the player must somehow get one of the monsters to melee attack and hit the other. This may be possible with the help of the invisibility power-up, which causes enemies to fire or attack in the wrong direction. Monsters will also go after monsters (including homogeneous ones) that have caused them indirect damage by exploding a nukage barrel. In version 1.1 of Doom a monster that had caused itself damage by destroying a nearby barrel would commit suicide.

Although a given species of demon is immune to its own type of projectile attack, they still employ it with its customary frequency while targeting one another. The immunity also means that certain species, such as Mancubi, cannot actually inflict damage no matter how many times they attack one another (except indirectly, e.g. by destroying additional barrels).

Monster infighting has become a novel aspect of Doom and its sequels, add-ons, and spin-offs. Indeed, most games do not give the enemies the ability to hurt each other, much less the ability to retaliate. In more recent first-person shooters, however, enemies are intelligent enough not to fire if a fellow enemy is in the way.

Causing infights has become a tactic in some of the more difficult user-made levels, such as those in Alien Vendetta. Often health and ammunition are so scarce that causing infights between monsters greatly reduces the burden on the player.