A mercenary is an irregular combatant partaking in armed conflict in exchange for monetary compensation. A mercenary is very rarely affiliated with one country; typically, a mercenary would affiliate with the highest-paying side.
Some mercenaries can be part of a company that offers mercenary services, usually known as a private military company or PMC. They are formally recognized as their own entities and can assign specific mercenaries for certain tasks.
In the mid-1990s, in response to Belkan threats of war, the Ustio Air Force began hiring mercenaries to improve its developing military. One particular unit was the 6th Air Division which had multiple mercenary squadrons under its command, including Galm Team and Crow Team.
Galm and Crow Teams became vital elements in the Belkan War of 1995. Galm, particularly its squadron leader Cipher, rose to fame on the front lines and became known by allies and enemies alike. Galm's efforts are attributed as a major factor in the Allied Forces' victory over Belka and the terrorist organization A World With No Boundaries.
Ustio was not the only country to hire mercenaries prior to the Belkan War. Dominic Zubov was first hired by the Yuktobanian Air Force in 1985 to help settle territorial disputes; he was then hired by Romny's air force in 1987, by Valka's air force in 1989, and finally by the Belkan Air Force in 1990. While serving for Belka, he was assigned the task of shooting down pilots deserting the country's air force.
In 1998, when the Usean Rebellion broke out, the Usean Allied Forces initiated Operation Fighter's Honor, where they would hire three mercenaries to be part of the newly-formed Scarface Squadron: Phoenix, John Herbert, and Kei Nagase. Their efforts, combined with that of the non-mercenary members of the Allied Forces, eventually brought the rebellion to an end.
In the world of Ace Combat: Joint Assault, Martinez Security is a private military company where Antares Squadron belongs to. It is reputable enough to be given special permission to take part in military exhibition with Seventh Fleet and the SDF. With the emergence of Valahia, Martinez Security joined up with other PMCs to form International Union Peacekeeping Force against this threat.
After the collapse of Valahia, many mercenaries were hired by Andre Olivieri to carry out his Golden Axe Plan. They collectively became known as the Golden Axe Plan Private Army (GAPPA). Martinez Security fought and stopped Oliveri's army in clandestine operations in the United States, culminating in a decisive battle between the Antares Squadron and the GAPPA, commanded by former Martinez Security pilot Milosz Sulejmani and his Varcolac unit.
Within the Horizon universe, mercenaries from Russia were hired by the Blatnoi organized crime group with the goal of instigating a regional revolt in East Africa. Blatnoi aided the local rebels with military training and provided weapons. Andrei Markov, the leader of the New Russian Federation's air force, left the Russian Air Force and became a freelance pilot sometime before the 2015 coup.
In the world of Ace Combat Infinity, the global military budget cuts caused by the Ulysses Disaster triggered a boom in the private security contractor industry. Several armed groups were hired by the United Nations Security Council, forming the UNICS (United Nations Independent Commando Services). As well, the overabundance of aircraft after the introduction of the Advanced Automated Aviation Plant caused several former pilots to become hired soldiers, giving rise to groups such as Martinez Security, Axe & Hammer and Arrows Air Defense and Security.