Fatigues are part of the Battle Dress Uniform worn by military personnel of the UNSCDF during combat duties. Rather than full-body combat armor, like those worn by the ODSTs, regular fatigues are relatively simple cloth garments, with the benefit of flexibility and light weight, though at the cost of less protection from enemy fire.
Though it provides very little actual protection, it can be covered with ballistic armor plating, such as the M52B Body Armor, and can also include camouflage to allow the wearer to blend into their environments. Known patterns are a gray "urban" color, a mottled green "jungle/forest" camouflage, and a light grey/white "snow" camouflage. While not nearly as effective as Covenant active camouflage, or Spartan-III SPI suits, it provides users with a fairly decent amount of concealment from enemy forces.
Fatigues are worn on missions where the added protection of heavier armor is a disadvantage, such as difficult terrain or for stealth missions. The clatter of armor may alert previously oblivious enemies to their approach, and the weight and bulkiness of the armor may inhibit the wearer's combat performance.
Likely to consist of normal cloth, rather than heavy armor plating, fatigues seem to provide little protection, especially from Covenant Directed-Energy Weaponry, which can melt and burn through most materials easily anyway. Advances in fabric materials and manufacture may allow the cloth a Kevlar-like resistance to projectile weapons or high durability, but its chief advantage lies in its ability to provide a steathier and more agile wearer.
Halo: Combat Evolved is the only known game to feature UNSC Marines wearing only regular fatigues and soft armor, without the overlaid rigid plating, especially in the mission 343 Guilty Spark. This does not alter their abilities, but Marines without rigid plating can take only half as much damage as the Marines wearing it.
- Oddly, if one melees a Marine wearing fatigues without armor, it will still produce the metallic clang just as it would on the armored fatigues. It's possible that the un-armored fatigue has some kind of ceramic plating inside or underneath the outer fabric, or may have just been overlooked by the developers.