This is the background for the campaign mode of the game, which in form is a first-person shooter if played cooperatively and a two-person shooter if played solo. In the local or online co-op mode, each player controls one character, and most of the game is indistinguishable from many other military-themed action shooters. In the solo mode, the player controls both characters, in much the same way that in other shooters the player has controlled squads of fellow solders, although in Ao2 the characters can combine for some interesting special abilities.

The gameplay mechanic that makes Ao2 different from other shooters is called aggro. This is a term borrowed from massively multiplayer online games in which the monsters are programmed to attack the character that is doing the most damage. Ao2 has a simple meter at the center of the screen that tracks which character is attracting the most aggro from the enemies. The theory is for one character to hunker down somewhere safe, attracting the enemies' fire, while the other character runs around, unnoticed, to the side and executes the enemies. In fact it is possible for one character to hold so much aggro for a sufficient time that the game will literally make the other character invisible.

Maybe a little too buddy-buddy

Despite the many attempts to differentiate them through gameplay and graphics, first-person shooters always have been and always will be about who the player's character is and about who it is shooting. So right up front, Ao2 has a problem. Because however much fun it is to use one character to boost another character up onto a balcony, it is hard to get behind the idea that it is better to kill al-Qaeda terrorists for cash than it is to do it as a sworn soldier of the U.S. military. Somehow the mix of a real-world enemy, a real-world war and two unkillable mercs in hockey masks just isn't as compelling as, for example, Call of Duty 4. The two games have similar plots, but CoD4 has the player killing for something besides the money to buy gold plating for his guns.

The guns in Ao2 are actually some of the game's best parts. There is a wide variety of them, and they each behave in slightly different ways. In addition, they are all very customizable, so that the player can buy different stocks, different barrels, larger clips and attachment shotguns and grenade launchers for them. The money to buy and upgrade guns is earned by accomplishing objectives in the levels, and the action occasionally pauses in the middle of a mission to let the player go shopping. There are only two downsides to the gun-buying system. One is that it is all the players can do with their hard-earned cash. Two is that there is no "try before you buy." So the only way to tell whether the character is better off pimping the gun it has or buying a completely new one is to play the game enough to earn money enough to do both.

In order to implement all the fancy two-character tricks, Salem and Rios spend a lot of time in each other's arms. They parachute together, they crouch behind shields together, they treat one another's injuries, etc. They are wearing lots of leather. And hockey masks. Even taken all together, it is less homoerotic than professional wrestling, but the vibe is strong enough that it deserves mentioning.

All in all, Army of Two is a competent shooting game for players who like to kill "real" people with "real" weapons, but it's at its best in co-op mode when two real friends are playing the two virtual ones. Unfortunately, having two players makes the aggro system and the two-character tricks, which are the heart of the single-player game, unnecessary.