The previous Ace Combat games occur in imaginary realms, whereas Ace Combat: Joint Assault is set in the real world. While the missions are set in real-world locations such as London, Tokyo, and San Francisco, they involve fantastical aspects such as enormous flying fortresses. The player assumes the position of a teenage pilot who joins the private military unit Martinez Security and is soon tasked with combating the Valahia, a type of robbers that threaten entire cities; however, the story quickly takes a turn for the more convoluted as the player progresses through the game.
The gameplay is almost identical to its predecessor, Ace Combat X: Skies of Deception, released for the PSP. The player takes control of a combat aircraft (either from a third-person perspective or from the view of the cockpit) and engages in dogfights with other aircraft (or other giant flying foes) while avoiding their missiles. After completing missions, which typically involve defeating a predetermined number of enemies or bosses, the player is rewarded with money, which can then be put toward the purchase of additional aircraft or the enhancement of existing aircraft. These aircraft are based on (and licensed from) real-life models, and each comes with its own benefits and drawbacks. For instance, specific models are better suited for combating foes in the air; other bombers are more successful against targets on the ground. The mission tree contains several forks, and thus the player will frequently have to decide between several objectives.
The only significant new additions are the “enhanced combat view” (the game zooms in to provide a closer visual look at the adversaries) and the “co-op mode,” which makes it possible to play the entire campaign alongside other players. These capabilities were added because modern air combat is typically waged across great distances. When this happens, the game uses branching missions, which means that the players—up to a maximum of four—are split into two teams. The remaining multiplayer modes are the same as the Skies of Deception mode, except they each support up to eight players instead of only four.