With the exception of Ace Combat: Joint Assault, Ace Combat: Assault Horizon, and Ace Combat Infinity (all set in the real world), the franchise takes place in an Earth-like world dubbed Strangereal, with different geography and countries.
There are four major landmasses in Strangereal where most Ace Combat games are set and are separated by several major oceans and large seas, called Osea, Anea, Verusea, and Usea.