For Machiavelli they were the scum of the earth — ill disciplined, fickle, cowardly and insubordinate. Mercenaries, he said, would never make a prince secure or defend the state against attack. And history has generally concurred with his judgment. The mayhem in the Congo in 1960 was exploited by notorious “dogs of war”, white buccaneers who kept the conflict going for years to enrich themselves and their paymasters. Most of these adventurers came to a sticky end: the most notorious, Bob Denard, fanned a series of uprisings across Africa, including four coups in the Comoros that all ended in farce and failure.

Simon Mann, reputed leader of the abortive attempt to overthrow the corrupt Government of Equatorial Guinea in 2004, could have stepped from the pages of Frederick Forsyth. Indeed, the coup planned by about 60 white mercenaries was almost a recapitulation of The Dogs of War, even including the choice of the former Spanish colony in the Gulf of Guinea. He paid dearly for his derring-do: four years jail in Zimbabwe, extradition to Equatorial Guinea and a 34-year sentence in the notorious Black Beach prison.

His release yesterday was a welcome humane gesture by President Obiang. But though there is no place in the modern world for adventurers paid to provoke coups, there is a growing demand for Mr Mann’s earlier services in running private security firms to guard diplomats and businessmen in dangerous locations. The job is tough, the pay good and the distinction between guard and mercenary fine. But when such former soldiers take the law into their hands, as Blackwater did, notoriously, in Iraq, bloodshed ensues. Machiavelli’s warning is no less relevant today.