Co-op shooters encourage squad-level solidarity but also normalise the idea of arbitrary, let's-you-and-him-fight Punch-and-Judy battles, much like the violent team sports that elite private schools force on their students, the future military officer class, to habituate them to immediate commitment to pointless wars of aggression. "America's Army", the highly dodgy recruitment tool developed and released by the US Army, has the best immanent critique of this. Players participate in a battle between US soldiers and "terrorists" who carry Kalashnikovs and don't wear uniforms. But obviously the game designers couldn't have players identifying with the terrorist team or having fun blowing up Yankee soldiers -- so whichever team you join appears on your computer as the US Army, and your enemies as the terrorists. When a player kills an enemy terrorist (who sees him/herself as an American soldier), they can pick up the corpse's AK-47, but the instant they touch it the gun will metamorphose into an M16. The game, to avoid implying that it's fun to be a militiaman killing Americans, ends up making the argument that there is no serious distinction between the United States and its "terrorist" enemies beyond superficialities of clothing and quality of equipment.
Cheers
CWS