A mercenary is an irregular combatant partaking in armed conflict in exchange for monetary compensation. A mercenary is very rarely affiliated with one country; they usually affiliate themselves with the highest bidder.
Private security contractors, one type of mercenary, are formal armed services offering services in exchange for payment. They usually have multiple mercenaries working for them, and specific ones are called on to take care of a certain task.
Several mercenaries were hired by the newly founded Ustio in 1995 during the opening of the Belkan War to replace the losses of their air force. One of these squadrons, the Galm Team, became a vital element in the expulsion of Belkan troops from Ustio and the Allied Forces' eventual victory, along with the suppression of the A World With No Boundaries terrorist organization. In particular, Galm leader Cipher gained much fame on the front lines, becoming a known figure between foe and friend alike.
Ustio was not the only country to hire mercenaries. One of the notable mercenaries not working for Ustio was Dominic Zubov, who, after seeing large bloodshed during the Tyumen Dispute and the Blood War, became a mercenary for the Belkan Air Force. He was known as an "escapee killer" and was given contracts to shoot down pilots escaping the Belkan 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.