Single Player-  The Army consists of Razor Squad, Delta Squad and Mustang Squads. You can have primary guns, secondary guns but no skills.
Razor squad is a four man squad. It consists of Sgt. McCarthy, Sgt. Anderson, Sgt. Rollins and Warrent Officer Starks. They mostly uses shotguns, and smgs.
Only appears in Oil Slick. Consists of "chief" (the player in Modern Combat: Sandstorm) Sgt. Rodriguez, etc. Mustang Squads: There are multiple squads in Mustang. Mostly you play with Mustang 1-4. The only other squad is mustang 1-6 and we know that because of this dialouge. "This is Mustang 1-4 for Mustang 1-6"
Multiplayer- The army in multiplayer is different from resistance in thes ways-