In the 14th century there were two pandemics. One was the Black Death, the other was the commercialisation of warfare. Mercenaries had always existed, but under Edward III they became the mainstay of the English army for the first 20 years of what became the Hundred Years War.

Then, when Edward signed the Treaty of Bretigny in 1360 and told his soldiers to stop fighting and go home, many of them didn't have any homes to go to. They were used to fighting, and that's how they made their money. So they simply formed themselves into freelance armies, aptly called ''free companies'', that proceeded around France pillaging, killing and raping.

One of these armies was called the Great Company. It totalled, by one estimate, 16,000 soldiers, larger than any existing national army. Eventually it descended on Pope Innocent VI, in Avignon, and held him to ransom. The pope made the mistake of paying off the mercenaries with huge amounts of cash, which only encouraged them to carry on marauding. He also suggested they move on into Italy, where his arch-enemies, the Visconti, ran Milan. This they did, under the banner of the Marquis of Monferrato, again subsidised by the pope.

The nightmare had begun. Huge armies of brigands rampaging through Europe was a disaster second only to the plague. It seemed as if the genie had been let out of the bottle and there was no way of putting him back in. Warfare had suddenly turned into a profitable business; the Italian city states became impoverished as taxpayers' money was used to buy off the free companies. And since those who made money out of the business of war naturally wished to go on making money out of it, warfare had no foreseeable end.

Wind forward 650 years or so. The US, under George W. Bush, decided to privatise the invasion of Iraq by employing ''contractors'' such as the Blackwater company, now renamed Xe Services. In 2003 Blackwater won a $US27 million no-bid contract for guarding Paul Bremer, then head of the Coalition Provisional Authority. For protecting officials in conflict zones since 2004, the company has received more than $US320 million. This year the Obama government contracted to pay Xe Services a quarter of a billion dollars for security work in Afghanistan. This is just one of many companies making its profits out of warfare.