Last Sunday Isis raised their black flag over Palmyra. Below the flag, in the days that followed, the usual carnage began: beheadings, torture, desecration. Syrian state TV has reported that over 400 civilians have been killed already, and the big question globally has become: how could this have happened? What went wrong with the Iraqi and Syrian troops? Isn’t there anything the West can do?

Lord Dannatt, the former head of the army, has called on the British government to ‘think the previously unthinkable’ and send troops. He’s right that air strikes are no substitute for decent ground troops — but he must also know there’s no appetite here or in America for risking our boys’ lives.

Perhaps, though, there’s another way of getting well-trained boots on the ground. If we want answers about how to squash Isis, we should look to another field of combat, 5,000 miles away from Palmyra — to a war has been all but won against equally determined Islamists.

After five grim months as part of Boko Haram’s self-declared ‘caliphate’, life is slowly returning to normal to the Nigerian town of Michika. Residents who fled in droves are trickling back to plant crops before the rains, and despite the desecrated churches and tales of neighbours kidnapped and murdered, there is optimism.

Also looking cheerful for the first time are local army units, who unlike the Iraqi army, have found unexpected success in pushing Boko Haram from Michika and other north-east Nigerian towns. They weren’t always this upbeat. This was the same military that failed to stop 276 schoolgirls being kidnapped from the nearby village of Chibok last year, and who fled when Boko Haram first rolled into Michika in the autumn.

So why the turnaround? In Michika recently, I found a clue. Down a dusty road, by a road block of empty munitions cases, I came across a gaggle of Nigerian soldiers, who angrily shooed us away when they saw our cameras. Before they did so, though, my photographer glimpsed six uniformed white men behind them, who made themselves scarce sharpish. Were they the SAS, perhaps? Unlikely. While both Britain and America gave ‘technical help’ to the Nigerian army after the Chibok kidnappings last year, they stopped short of providing special forces. Instead, the most obvious explanation was that they belonged to another, equally publicity-shy force: mercenaries.

This is not simply an educated guess. As my newspaper, the Telegraph, reported that same week, in desperation the Nigerian government engaged a firm of South African guns for hire in January, going by the rather sinister name of Specialized Tasks, Training, Equipment and Protection, or STTEP. It’s run by Eeben Barlow, a former colonel in the South African Defence Force, whose men have a wealth of bush war experience.

In Nigeria, as it turns out, they have not just been ‘advising’, but getting the job done — flying attack helicopters, gathering intelligence, training troops — everything, in other words, that the army’s bloated high command proved so lousy at.

How the press wailed about Boko Haram before Isis dominated the news, yet the defeat of one of the most vicious Islamist insurgencies in modern Africa has gone all but un-remarked. STTEP’s men have not been hailed as liberation heroes or thanked by the UN. More importantly, in an era where the West is increasingly reluctant to carry out ‘outside intervention’, no one seems to be asking whether this model could be used in other desperate countries.

This is in part because the Nigerian government wants to keep quiet about having to rope in help, especially from white South Africans. But it also reflects a lingering sense that using private soldiers is ethically questionable, no matter how urgent the need for them.

Barlow’s outfit is made up of black African soldiers as well as whites, and even includes ex-communist guerrillas. Yet the combination of private money and apartheid-era expertise remains a toxic one in many people’s minds. Hence the calls for STTEP’s prosecution when reports of its involvement in Nigeria first emerged earlier this year, and hence Col Barlow’s indignant retort: ‘I am proud that my “racist”, “mercenary” group of trainers added value to the Nigerian army’s fight against terrorism.’

Indeed, this isn’t the first time his firm has helped an African government quell an otherwise unstoppable insurgency. His previous venture was Executive Outcomes, which helped Sierra Leone fight off the drug-crazed, limb-chopping rebels of the Revolutionary United Front in 1995.

Back then, amid a chorus of international disapproval at the presence of guns for hire, they were replaced by vastly more expensive and less competent UN-backed peacekeepers, who allowed the RUF to ransack Freetown again. Peace was not restored until 2000, when Tony Blair despatched a force of British paratroopers to run the RUF out of town.

So could private military companies, or PMCs, become part of a potential solution for Syria and Iraq? I can well imagine that they might have been effective early on, back when the Free Syrian Army was still the biggest gang in town. They could have provided high-class on-the-ground advice, logistics and training to what was otherwise a ragtag army of farmers and disaffected conscripts. Now, if allowed, they could certainly tip the odds in Isis-held territory. In both Nigeria and Iraq, the reason troops so often flee is not because they’re outgunned, but because rampant corruption and appalling management leaves soldiers unpaid, under-fed and short of ammo and functioning armoured vehicles.

Fighting the fanatics of Isis would certainly carry a high risk of PMC casualties. Yet this was normal in the second Iraq war, where thousands of former British and American servicemen were employed. Some 225 private military contractors died in Iraq between 2003 and 2011 — more than the entire British loss of 179 soldiers.

The other issue is the UN, which in 1989 set up the United Nations Mercenary Convention, which prohibits their use, although Britain and the US are not among its 33 signatories, and dispute its definition of a PMC. We should remember that UN peacekeeping missions have a lousy track record. According to Colonel Tim Collins, the Iraq war veteran who runs New Century, a PMC that does military training in Afghanistan, they are usually blighted by ‘toothless contingents, indecisive command and the distinct stench of corruption’. But he points out that any challenge to their monopoly would run up against not just ideological objections, but self-interest, given the vast amounts of money paid to developing nations that field peacekeeping units.

‘It would need a level of co-operation from the UN that currently does not exist,’ he told me. ‘But while 20 per cent of that is a Guardian-ista sniffiness about PMCs, 80 per cent of it is because fielding UN forces is a great scam for many countries.’

There would have to be restrictions as to whose side PMCs fought on — few would want STTEP fighting for Robert Mugabe, for example — but surely a clear licensing system could be established. Besides, the more respectable PMCs have long moved on from the ‘Dogs of War’ image of the 1970s. The bigger operators have ex-generals and ministers on their staff, who would be wary of a client that brought them into disrepute.

Efforts could be made to make their activities more transparent, with less hiding behind ‘commercial confidentiality’ clauses. They might also think about rebranding. Shadowy sounding names like ‘Executive Outcomes’ make PMCs sound like Bond villains, when in fact they’re often rightly proud of the work they do. The more transparent the industry becomes, the better. Who knows, one day I might bump into a mercenary in some distant warzone who won’t slink away, but can be open about what he’s achieved in places where others failed or didn’t dare to go.