Private Military Contractors / Real Life

Ancient

• Mercenary work was popular among ancient Greeks, where mercenaries were known as misthophoroi. The island of Crete in particular is noteworthy, as the Cretans were highly skilled with the sling and hired themselves out to provide reliable long-range support for Greek and other regional armies.
• Ancient Hispania, especially regions like Celtiberia, Lusitania and the Balearic Islands, was a veritable gold mine of mercenaries, as the unequal control of livestock and fertile land meant than large sectors of the population were essentially left to starve to death unless they left to find a job elsewhere - and as many of them had no other skill or craft than having been exposed to tribal warfare since they were old enough to lift a sword, this is what they would do for a living abroad. The nearby Merchant City of Carthage was especially fond of hiring them, as those social conditions meant you could raise instant armies of battle-hardened Hispanics with minimal effort if you had the right connections and amounts of gold, and at some points of the Punic Wars the city depended almost entirely on them to defend their territories in Hispania and Africa. Only after Hispania was entirely Romanized this custom died off, and it still left quite a mark in Rome, with the excellent gladius sword being adopted from Celtiberian mercenaries after those massacred too many Romans with them.

Medieval and early modern

• El Cid Campeador might be one of the most successful examples in history, even if not many people know he and his followers worked as mercenaries at all. He served both Christian and Muslim clients before eventually becoming an independent warlord and planting his own protectorate in Valencia.
• Another famous example is the Almogavars, infantry shock troops recruited all over the Iberian kingdoms with an origin in the Kingdom of Aragon who were feared for their ferocity and their Battle Cry, "desperta, ferro" ("Awake, iron!"), which they would shout as they struck their weapons with a piece of flint, making enormous sparks. A group of them led by Roger de Flor formed the Catalan Company, which was hired to help protect the Byzantine Empire, which they did with great success, but then their leaders were murdered by orders of the Emperor — which led to the Catalan Revenge, a two-year-long war of revenge and looting. The Almogavar survivors would later move to the Duchy of Athens, but after being denied their pay they went and conquered both the Duchies of Athens and Neopatras.
• The Medieval Italian compagnia di ventura or companies of adventure, which were basically private mercenary armies ready for any city state to hire. In the thirteenth and fourteenth centuries, the Italian city-states of Venice, Florence, and Genoa were very rich from their trade with the Levant, yet possessed woefully small national armies. In the event that foreign powers and envious neighbours attacked, the ruling nobles hired domestic and foreign mercenaries to fight for them. The military-service terms and conditions were stipulated in a condotta (contract) between the city-state and the soldiers (officer and enlisted man), thus, the contracted leader, the mercenary captain commanding, was titled the Condottiere. The mercenaries then formed private companies and armies which sold their services to the states. The condottiere armies got much criticism from Niccolò Machiavelli, but in the Real Life they proved to be more competent than usually imagined. (Of course, Machiavelli's real concern wasn't so much with their effectiveness as fighters as the fact that the state's military force was a heavily armed gang loyal only to their last paycheck, thus creating instability; Machiavelli was also a big believer in the ability of a citizen-army to instill virtue in the population, so that had an effect as well. Basically, Machiavelli wanted a state to rely on its own people to fight its wars because they had skin in the game and because having the people defend the country encouraged patriotism.)
• For several hundred years, the Reisläufer or Swiss mercenaries were considered to be the finest soldiers in Europe. They even wore garish outfits so that everyone on the battlefield would know who they were. This is why the Vatican has the Swiss Guard, and why they wear such funny uniforms. Swiss mercenary companies would not fight an enemy force that included another Swiss mercenary company, so national loyalty was apparently more important than money in this case.
• Another famous Renaissance-era group of mercenaries were the German Landsknechts, who were modeled after and considered the primary rivals of Swiss mercenaries. Created by Maximilian I, the Landsknechts used a combination of halberds and pikes, much like the Swiss, but also employed a comparatively larger number of early firearms and artillery. They were also famous for the zweihander, a sword that could be as long as six feet designed to be used like a halberd to fend off multiple enemies and to obstruct enemy pikes. They did not share the Swiss's scruples about fighting their countrymen — or for that matter, doing anything like protecting the papacy, the last many of them were Protestants. Maximilian's grandson Charles V used them in plenty, but thousands of them became mutinous and sacked Rome in midst of the campaign in 1527 after they were not paid (which they did after killing the hopelessly-outnumbered Swiss Guard who stood behind to help the Pope flee to a nearby castle). The Spanish Empire continued hiring them, although their prestige would gradually over the next century until disappearing.
• Ambrogio Spinola entered The Eighty Years' War as a mercenary in the service of the Spanish Empire. Ironically, King Philip III's lack of funding and central authority ended up reversing things and turning Spinola into a sort of warlord (officially called "proconsul"), forcing him to pay his tercios off his own pocket, live off the country, and operate independently from Madrid. He eventually got tired of this and quit.
• The Portuguese Empire had the African Imbangala, Angolan warrior brotherhoods who served as mercenaries and slave hunters. The Portuguese employed them as native force in campaigns such as those against the kingdom of Ndongo.
• Another common source of mercenaries during 17th and 18th centuries was Ireland. Most European Catholic nations during this period had a few units of Irishmen in their armies. This was actually encouraged by the British as every young Irish male with military skills remaining in Ireland was a potential rebel! Descendants of these Irish mercenaries would become heads of states in France (Patrice de MacMahon) and Chile (Bernardo O'Higgins) even before there was an independent Ireland.
• For much of European history, especially in times when Patriotic Fervor was a smaller consideration, it was not considered dishonorable to fight in a foreign army as long as one does not fight against one's own government. When a petty-noble was left unemployed, he would often look for work for a foreign ruler. If he was promoted enough he might even change countries and be remembered in history more by the country he fought for then by the country he was born in.
  • Baron Friedrich Wilhelm von Steuben was a Prussian military officer who was unable to find work in Europe and ended up joining the Continental Army in The American Revolution- though as a volunteer, as nobody could afford to pay him. His greatest contribution to the war effort was to reform the Ragtag Bunch of Militia into something resembling a real disciplined army, and as such is regarded as one of the fathers of the US Army. After the war, he retired and was given estates in New Jersey and New York in thanks for his service.
  • Hans Axel von Fersen was another who served in the American Revolution, as seen in The Rose of Versailles.
  • John Paul Jones was a Scotsman (and very much anti-English at that) but is most famous for his service to Russian Empire and the United States.
  • Prince Eugene of Savoy was Italian but spent his life in the service of the Habsburg monarchy.
  • Even Napoléon, merely an Artillery captain at the time, considered enlisting to the Russian Army at one point, but decided against it so as not to lose seniority because there was a mandatory rank penalty for the foreign officers at the time.
  • Many of the minor German states that made up the Holy Roman Empire engaged in the Soldatenhandel ("soldier trade"), renting out their own organic military forces as auxiliaries (technically legally distinct from mercenaries, but similar in spirit) to keep their skills sharp during peacetime and bring in a little extra cash. The most famous of these were the "Hessians" who fought for the British in the American Revolution, though the term was often broadly used to refer to any German national who fought alongside the Redcoats (including those from the Electorate of Hanover, who weren't even auxiliaries as they were fighting for their legal sovereign the Elector, a.k.a. King George III of Great Britain).
• In the years leading up to The French Revolution, one of the French king's Household Cavalry regiments was known as "Le Royal Suedois" (The Royal Swedish) because all its officers and the bulk of its enlisted men were expatriate Swedes, while the infamous "Royal Allemand" used in the attempt to repress the riots of Paris had its officers and enlisted coming from Germany.

Modern

• A lot of modern military contractors, at least those operating in Western countries, are somewhat of a subversion of the trope in the sense that they are not really capable of doing what a real military does: they are more of glorified, heavy-duty security guards without the means to engage in "real" combat where heavy military equipment is involved. The Other Wiki has a list of some of the most notable examples. This may be in part because of the United Nations Mercenary Convention, which prohibits the "recruitment, training, use, and financing of mercenaries". Using private military contractors for offensive operations would arguably be a violation of international law. However, China, France, India, Japan, Russia, the UK, and the US are not signatories to the treaty.

Currently active

• Academi, formerly Xe Services LLC, Blackwater Worldwide, and Blackwater USA. Should be noted that it's their actions, command structure, and actions of their chairman and CEO, Erik Prince (a former Navy SEAL), that separates themselves from more mundane contractors. That, and the mind-boggling extent of both the access to equipment and the variety of businesses that they have. Ironically, though they were perhaps most notorious for their security work (and allegedly fighting alongside coalition military forces in Iraq in the 2004 Battle of Najaf), their chairman/CEO would eventually announce a scaling back of that work (due to the criticisms) in favor of the other services. Blackwater's also interesting in that they're one of the few mercenary companies to have been deployed in America. After Hurricane Katrina, they actually arrived ahead of the National Guard.
• Draken International is a private air force that operates a variety of fighter jets, primarily for aggressor training. Sadly they don't have any actual Drakens in their fleet.
• Cohors pedestris Helvetiorum a sacra custodia Pontificis, better known as a Papal Swiss Guard, is technically the last one of the legendary Swiss Mercenary Companies mentioned above — in fact, when the modern Swiss constitution finally forbade hiring out Swiss nationals as mercenaries, it explicitly created an exemption for the papal guards.
• G4S deserves special mention for being the 3rd-largest private employer in the world as of 2012 (and have reached 2nd place as of 2016, behind only Walmart).
• This video provides a quick overview of the 7 largest PMCs in the world as of 2016. Both Academi and G4S are on that list.
• Chinese PMCs are rather new: private security companies were heavily restricted in China until the government relaxed the relevant laws on January 1, 2010. Since then, they have been very active in recent years; this is a result of them heavily investing in African infrastructure (e.g. hospitals, airports, schools, mines) in some extremely volatile places like Sudan. One of the bigger names in this industry is Shandong Huawei Security Group. The "One Belt, One Road" initiative to open up new trade routes for China across Central Asia and the Indian Ocean has also proved to be a boon for Chinese security services providers.
• The Wagner Group is often used by the Russian government to provide forces for allies in conflicts where it suits them to have plausible deniability. They've been active in Eastern Ukraine, Syria, Sudan, and the Central African Republic in the late 2010s and early 2020s. They are notable for having been brazen enough to launch an attack on a US base in Syria... which went poorly for them. After years of denying his involvement, Russian oligarch Yevgeny Prigozhin finally admitted in September 2022 that he founded the Wagner Group in 2014 to support the Russian forces in Eastern Ukraine. This was due to a leaked video of Prigozhin offering prison inmates freedom if they served for six months in the 2022 Russian invasion of Ukraine, in which the group has seen its heaviest involvement yet, in fierce battles of attrition such as Bakhmut. It is through this recruitment of convicts that the Wagner Group has inflated in size from 5,000-10,000 to nearly 50,000. Wagner is somewhat unique among modern mercenary groups in that it has a lot of heavy equipment, including tanks and fighter-bomber aircraft. It is widely seen as a state-supported enterprise through Prigozhin's close association with President Vladimir Putin. However, on July 23, 2023, after a period of increasing tensions between Yevgeny Prigozhin and the Russian Ministry of Defence, the Wagner Group launched an abortive rebellion against Defense Minister Sergei Shoigu and Chief of the General Staff Valery Gerasimov which ended in the exile of Prigozhin and the Wagner Group to Belarus. Exactly two months after the rebellion, Prigozhin was killed in a plane crash near Kuzhenkino in Tver Oblast under suspicious circumstances.

Historical/Defunct examples

• A well-known example is the private firm Executive Outcomes, which contracted its former South African soldiers to win wars, not fight them. The firm, based in Africa, was responsible for several contracted wars, in which other firms that were owned by EO's parent company gained access to oil fields, diamond mines, and the like. Executive Outcomes folded in 1999, but the majority of its operators and equipment were shuffled around into other private military firms owned by the parent corporation. To add to the irony, while most of EO's members (at least initially) were ex-military of the Apartheid Era South Africa, one of their first clients was the anti-apartheid MPLA government of Angola against which they had fought during the Apartheid. EO was even pitted against their former allies, the UNITA rebels. Money is God indeed.
• UK firm Sandline International was another case of a real-life Private Military Company and had close links with Executive Outcomes. After it shut down, most of the staff went on to form Aegis Defence Services. Despite going out of business on April 16, 2004, its company website remained accessible for an impressively long time until it finally went offline for good at the end of 2020. Their involvement in the situation in Sierra Leone, including the coup against Kabbah, was a source of embarrassment to former Foreign Secretary Robin Cook in 1998.
• Mercenary armies were used during the Congo Crisis and Biafra War, with several mercenaries rising to fame such as Bob Denard, 'Mad Mike' Hoare and Rolf Steiner.
• The Flying Tigers were American pilots who were effectively hired out to the Chinese to fight Japan shortly before the outbreak of World War II.
• In the Spanish Civil War, both the Loyalists and Nationalists employed mercenaries (particularly the Condor Legion, which bore the same relationship to the Nazi military machine that the Flying Tigers bore to the US military). Fighter and bomber pilots especially were highly sought-after.
• George MacDonald Fraser, in Quartered Safe Out Here, tells of a British officer who recruited local tribesmen to harass the Japanese during WWII. Similar types were generally known in the partisan war and espionage game as they always have been.
• As late as World War I, it was considered acceptable for officers to take a leave of absence to go off and fight in someone else's war, provided one didn't fight for one's own country's enemies or their allies. It was considered a valuable way to inject some combat experience into what would otherwise be a purely peacetime officer corps, as well as build up some goodwill on the sly, while still being technically a private venture, and as such completely deniable.
• In 19th and early 20th Century America, corporations would sometimes field a virtual army of mercenaries to break strikes and enforce their interests.
  • The Pinkertons were the most notorious and powerful example, to the point that they outgunned the U.S. military during the Homestead Strike, ultimately leading to the Anti-Pinkerton act in 1893.
  • Coal and Iron Police of Pennsylvania were another notorious example: although commissioned by the state legislature, they sold the right to appoint officers to individual mine and foundry operators (who would then hire whomever they wanted, usually thugs who would work cheaply). They were primarily used as particularly brutal strike breakers but had full arrest powers on company property. They also served as political muscle for the large industrial interests, which was a factor (along with a number of assaults, rapes, and murders) that led to their disbanding by order of the governor in 1931. The Coal and Iron Police existed alongside the Pennsylvania State Police (which initially only had jurisdiction over rural areas) for 25 years but had more effective power prior to that date.
• The Russian Civil War had various foreign mercenary units, sometimes formed from stranded POW or civilian workers and sometimes former Imperial regular units from seceded territories, who fought for the White and Red sides. The Reds had the Latvian Riflemen and the Chinese International Units. The Whites had the Czech Legion. Since the whole place and time was extremely lawless and unpleasant, most of them were just Bloody Mummers with guns.
• Foreign mercenaries were common in China between late 19th and early 20th centuries. With the regular armies in disarray, Qing government, at both national and local levels, hired numerous foreign mercenaries to lead its forces against various rebel forces. Some of these, such as American Frederick Townsend Ward, were genuine foreign adventurers seeking profit and adventure, while others, such as British Charles "Chinese" Gordon, were regular military officers instructed by their governments to help the Chinese government survive. After the empire fell and China was divided among numerous warlords, numerous foreign mercenaries were once again recruited to help these armies.
• Modern standing armies were originally collections of mercenary units under permanent contract to a given ruler (not always mercenaries strictly speaking; most were lawful subjects of the ruler, but they served under the same terms). They were usually raised privately by a local noble often from his neighbors. In fact, much of the modern traditional ranking system was originally commercial in concept. Colonel for instance originally meant "CEO of a regimentum(regiment), that is a mercenary band with a standing royal contract.
• One common variation known was for a ruler of a small country to simply "rent out" his army to belligerents. These were not strictly mercenaries; the soldiers in them tended to be enlisted in the forces of their lawful prince. However someone enlisting would likely know what he was in for. German states were famous for this; their soldiers were such a good buy and their rulers so willing to deal, and they were often times not busy with a war on their own account as major powers were, and so could spare troops. This is the origin of the infamous "Hessian mercenaries" in The American Revolution.
• The largest private armies in history were perhaps the Presidency armies employed by the Honorable East India Company of Great Britain, which essentially ruled the Indian subcontinent for roughly a century. They were eventually nationalized (along with the rest of the EIC's infrastructure) in the aftermath of the Indian Rebellion of 1857, itself triggered by a series of mutinies among the native soldiers.
• Soldier of Fortune magazine was infamous for its classified section where dozens of PMCs and armies around the world put out wanted ads for people who wanted to join up. Even more notorious were its "guns for hire" section, where would-be triggermen advertised their services. After several high profile contract killings that were eventually traced to these ads (as well as one small group of American and British mercenaries getting executed by the government of Angola for fighting on behalf of the FNLA), the magazine permanently shut down its classified section in 1989.