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 "free companies", that proceeded around France pillaging, killing and raping.
One of these armies was called the Great Company. It totalled, according to one estimate, 16,000 soldiers, larger than any existing national army. Eventually it descended on the pope, 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 that 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.