With their roots in South Africa's apartheid-era security forces, they don't fit the image of an army of liberation. But after just three months on the ground, a squad of grizzled, ageing mercenaries has helped end Boko Haram's six-year reign of terror in northern Nigeria.
Run by Colonel Eeben Barlow, a former commander in the South African Defence Force (SADF), the bush warfare experts were recruited in secrecy in January to train an elite strike group within Nigeria's demoralised army.
Some cut their teeth in South Africa's border wars 30 years ago. But their fighting skills, backed by their own helicopter pilots flying combat missions, have proved decisive in helping the military to turn around its campaign against Boko Haram. The Islamists have fled many of the towns they once controlled, leading to the freeing of hundreds of girls and women they used as slaves and "bush wives".
The role of Barlow's firm in tackling one of the most vicious insurgencies of modern times has been kept largely quiet by Nigeria's outgoing President, Goodluck Jonathan, who lost elections six weeks ago to ex-General Muhammadu Buhari. But last week Barlow, 62, described his "aggressive" strike force in a seminar at the Royal Danish Defence College and in an interview with Sofrep.com, a special forces website.
During apartheid, Barlow served with the SADF, a mainly white force that defended the regime. In 1989 he co-founded Executive Outcomes, a company made up of many ex-members of South Africa's security forces. One of the first modern "private armies", in 1995 it helped Sierra Leone's Government fight the rebels of the Revolutionary United Front.
Barlow's new company, Specialised Tasks, Training, Equipment and Protection, is thought to have sent about 100 men to Nigeria, including black troopers who served in elite South African units. Others fought as guerrillas against the SADF. It raises questions about the level of help the Government was receiving from the British and US militaries, which offered "mentoring" after Boko Haram's kidnapping last year of more than 200 schoolgirls from Chibok.
Barlow said the plan was to train a team to free the schoolgirls. As Boko Haram continued to massacre hundreds in village raids, they turned to schooling Nigeria's traditional army in "unconventional mobile warfare". The key was "relentless pursuit", which involved mimicking Boko Haram's hit-and-run tactics with nonstop assaults. Once the insurgents were on the run and their likely route established, the strike force would be helicoptered ahead to cut off escape routes.
STTEP used bush trackers to work out where their enemies were going, an art that proved vital in forest hideouts. "Good trackers can tell the age of a track as well as indicate if the enemy is carrying heavy loads, the types of weapons he has," said Barlow.
While the Nigerian Government has insisted STTEP were "technical advisors", Barlow suggested his men had been involved in direct combat.