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 Brétigny 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...