While the modern breed is more likely involved in logistics than combat, they are still armed civilians operating for profit and aren't recognized soldiers under the Geneva Conventions.
Peter Singer of the Brookings Institution said some of the PMSCs are operating at the "tip of the spear" in Africa and are directly descended from their mercenary forebears.
One catalyst for the mercenaries' re-emergence in their latest incarnation appears to be the U.S. Africa Command.
Inaugurated in October 2008, Africom's stated mission is to help train counter-terrorism forces across the continent where al-Qaida has gained footholds in East Africa, Somalia and North Africa.
But critics say its true purpose is to provide a framework to protect U.S. energy interests since West Africa is the world's hottest oil boom zone and is expected to supply about one-quarter of U.S. oil imports by 2015, which gives it strategic status.
Whatever Africom's mission, there are opportunities for PMSCs under lucrative U.S. outsourcing contracts, such as the U.S. State Department's Africa Peacekeeping Program, worth $1 billion over five years.
The program involves security, aerial surveillance, logistics, construction and training in countries like Somalia, Sudan and the Democratic Republic of Congo.
U.S. mercenaries such as Blackwater -- renamed Xe Services because of the bad name it got in Iraq operating as a U.S. contractor -- DynCorp and Triple Canopy have been eying Africa as the industry's new frontier, as it was in the heyday of the "Dogs of War" from the 1950s to the 1990s.
Indeed, U.S. reports in June 2010 said Blackwater Worldwide tried for two years to secure lucrative contracts in rebel-held southern Sudan even though the war-torn country was under U.S. sanctions.
In 2010, U.S. officials said Blackwater had at one stage proposed that secessionist southern Sudan, scheduled to become an independent state in July, pledge up to half its oil wealth to pay for Blackwater's services.
Africa's vast mineral riches have long been a magnet for the soldiers of fortune.
The eruption of wars across the continent in the post-colonial era were largely triggered by the greed of nationalist warlords -- and their foreign sponsors -- for gold, diamonds, oil, copper, timber and a host of other commodities.
In 2004, British and South African mercenaries sought to mount a coup to topple President Teodoro Obiang of oil-rich Equatorial Guinea.
The plot was organized by British financiers backing exiled opposition leader Severo Moto, who would award preferential oil rights to them once he took power.
The coup fell apart March 7, 2004, when a Boeing 727 carrying 67 mercenaries led by former British Special Forces officer Simon Mann was held in Zimbabwe en route for Equatorial Guinea.
Most of the mercenaries were South Africans, former members of the notorious 32 Buffalo Battalion, a Special Forces unit that fought for the apartheid regime. They were convicted and imprisoned in Equatorial Guinea.
Mann, a veteran of the 1991 Gulf War, was sentenced to 34 years in prison on July 7, 2008, but was released on "humanitarian grounds" Nov. 2, 2009.
He had earlier worked for Sandline, a South African security company set up by former British army officers that provided the template for the mercenary firms that emerged in the 1990s.
Sandline folded in April 2004. The best known of the new outfits was Executive Outcomes, based in South Africa where there was an abundance of military veterans following the apartheid era, as well as Europeans lured by Africa's seemingly endless wars.
In this period, most of the mercenaries in Africa were employed by regimes -- some of which they had once fought -- to protect their oil fields, gold and copper mines and other vital installations.
Executive Outcomes, for instance, provided former South African soldiers for Angola, which they had fought in the 1970s, to help its army regain control of the Soyo oilfields from rebels Pretoria had once supported.
Executive Outcomes was dissolved when South Africa banned its citizens from involvement in mercenary actions abroad in 1999. Most of Mann's men were EO vets.
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