Ace Combat: Joint Assault leaves the fictional worlds of previous Ace Combat installments and enters the real world. The missions take place over London, Tokyo and San Francisco, but still feature fictional elements like huge flying fortresses. The player takes the role of a young pilot who joins the private military force Martinez Security and soon has to fight the Valahia, some sort of bandits which threaten whole cities - but of course the plot soon becomes more complicated.
The gameplay is mostly identical to the PSP predecessor Ace Combat X: Skies of Deception. The player controls combat airplanes (either in third-person or from a cockpit view) and fights other airplanes (or other big flying enemies) while dodging their missiles. After finishing missions, which mostly consist of killing a certain number of enemies or boss fights, the player receives money which is reinvested into unlocking more airplanes or upgrades. These aircraft are based (and licensed from) real-life models and have certain advantages and disadvantages, e.g. some are more suitable against flying enemies while bombers are effective against ground targets. The mission tree is branching and the player has to choose between multiple missions at times.
The only major new features are the "enhanced combat view" (because modern air combat is usually fought over long distances, the game zooms in to give a nearer visual look at the enemies) and the co-op mode which allows to play the whole campaign with other players. Then the game takes advantage of the branching missions, meaning that the (up to four) players are divided into two teams. The other multiplayer modes are identical to Skies of Deception, except that they allow eight instead of four players to participate.