As different as each of them were, they all had one thing in common, at some point one side wanted more troops.
Most battles eventually come down to boots on the ground and rifles in the field. So when commanders are building their ranks it’s often with professional soldiers who know how to fight, and get paid well to do it.
The idea of a mercenary may seem a bit quaint in the 21st century, but those forces make a difference and are often all that stands between a leader and his fate.
With more than 625,000 employees, this listed security giant is the second-largest private employer in the world (behind Wal-Mart). While some of its business is focused on routine bank, prison and airport security, G4S also plays an important role in crisis-zones right around the world.
In 2008, G4S swallowed up Armorgroup, whose 9,000-strong army of guards has protected about one third of all non-military supply convoys in Iraq (it's also notorious for its wild parties and for having Afghan warlords on its payroll).
But the combined group has a security presence in more than 125 countries, including some of the most dangerous parts of Africa and Latin America, where it offers government agencies and private companies heavily-armed security forces, land-mine clearance, military intelligence and training.
With more than 1,200 staff worldwide, the Australian-owned Unity Resources has been able to grow its presence in Iraq as sovereign armies withdraw. Its management consists of veterans from Australia, the U.S. and Great Britain.
The private military firm is best-known for guarding the Australian embassy in Baghdad, where, as of 2010, it had trained Chilean soldiers to man gates and machine-gun nests. Unity personnel were also responsible for two controversial car shootings in Iraq: one killed an Australian professor, another resulted in the deaths of two civilian women.
Outside Iraq, Unity has assisted with security during parliamentary elections in Lebanon and helped evacuate private oil companies from crisis zones in Bahrain. The firm also operates throughout Africa, the Americas, Central Asia and Europe.
Erinys has also followed U.S. State Department contracts to Iraq. Its biggest mission in recent years took 16,000 of its guards to 282 locations around the country, where they protected key oil pipelines and other energy assets.
The group also maintains a presence in Africa, where it has traditionally focused its operations. Erinys was recently awarded two contracts in the Republic of Congo, for security at major iron ore and oil and gas projects.
Formerly owned by Hashmat Karzai, the first cousin of Afghan president Hamid Karzai, Asia Security Group is a major local force in the war-torn nation. It employs about 600 guards.
The private army, headquartered in Kabul, has been awarded millions of dollars in contracts from the U.S. military and is said to protect Coalition supply convoys travelling in Afghanistan's south. Mercenaries from Asia Security Group have also been recruited by DynCorp, a U.S.-owned contractor with a big footprint in the region.
DynCorp, based in Virginia, is one of eight private military firms specially chosen by the U.S. State Department to remain in Iraq as official American forces pull out.
But the huge group, which brings in about $3.4 billion in revenue every year, is also active throughout Africa, Eastern Europe and Latin America, with a staff in excess of 10,000. The firm earned a trigger-happy reputation as its soldiers fought rebel groups in Columbia in the early 2000s. Its troops have also engaged in anti-drug missions in Peru and were sent to disarm fighters in Somalia, Liberia and southern Sudan.
Another of the eight contractors recruited to replace official U.S. forces in Iraq, Triple Canopy has an army of about 1,800 troops in the country -- mostly from Uganda and Peru -- on contracts worth up to $1.5 billion.
An official review of the firm's team in Iraq concluded it was a 'well-trained, professional work force with significant prior experience.' But the private military -- whose name refers to the canopies in the jungles where its founding Army specialists received their training -- also employs another 3,000 personnel globally.
Contracts in other parts of the world have taken Triple Canopy to Haiti, where it guarded the U.S. embassy, and to Israel, where agents provided personal protective services for the U.S. State Department.
Aegis supplies forces for private clients, U.N. missions and the U.S. government, especially in Iraq.
But its staff, estimated to be as big as 5,000, is also spread across offices in Afghanistan and Bahrain, where the contractor offers emergency response, risk assessments, and protects private oil interests.
The private military contractor is probably best-known for a video that surfaced in 2005, which allegedly showed Aegis forces firing at Iraqi civilians.
In the past, Triple Canopy has recruited heavily from the ranks of Defion Internacional, which sources and trains private military personnel from Latin America for jobs right around the world.
Headquartered in Peru, and with offices in Dubai, Iraq, Philippines and Sri Lanka, the firm contracts and trains bodyguards, drivers, static guards and logistics specialists from a number of developing countries. In some cases, these agents are paid as little as $1,000 per month, which has drawn international ire -- especially for jobs linked to the U.S. State Department.
At one stage there were more than 1,000 Latin Americans guns-for-hire in the Middle East, although it is unclear how many of those fighters Defion was responsible for given that it is not required to disclose numbers.
Formerly Blackwater, then Xe Services, Academi runs a 7,000 acre training facility deep in the North Carolina wilderness -- one of the biggest and most complex private military training grounds in the world.
According to a book written on Blackwater in 2007, the facility had by then produced an army of 20,000 troops, 20 aircraft, a fleet of armoured vehicles and trained war dogs. Most of those resources were shipped to Iraq and Afghanistan on U.S. government contracts.
Academi probably scaled back after a number of wrongful shootings and other controversies angered the Iraq government and jeopardized important contracts.
Outside the Middle East, Academi was recruited to protect the streets of New Orleans after Hurricane Katrina. It has also protected Japan's missile defence systems and assisted with the war on drugs around the world.
Take a course at Academi's premier training facility in North Carolina.
The firm offers custom courses for allied security forces and corporates, such as live-fire driving instruction, counter-terrorism training -- including dealing with weapons of mass destruction -- and executive risk assessment.
You can also get equipped at the Academi web store, which stocks everything from protective sunglasses to sniper mission logs -- even branded gifts.
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