"Kill every living creature he came across" is dramatic hyperbole, but while Edward's armies might not have been slaughtering every frog and field mouse, they certainly would have stolen, killed, or eaten every horse, cow, sheep, chicken, and pig in the path of their march, as well as every deer, grouse, and rabbit they could catch. The human inhabitants of these areas would have fled to the nearest walled town at the first word that a hungry army was nearby. Those that remained faced the risk of assault, robbery, rape, and murder.

As Napoleon famously said, "An army marches on its stomach." The chevauchées employed by Edward III and his son were not new to the history of warfare, though it may have seemed novel in the age of castles and tit-for-tat siege warfare. The primary benefit of such a strategy was to liberate an army from the need for a cumbersome supply line, instead feeding the army on the bounty of the enemy countryside. An army of 10,000+ people and animals consumes literal tons of food every day, so the army had to keep moving to continue eating. While this meant that the army was very mobile, it also meant that it could not stop for long in any one place. Brief surprise attacks could be made against fortified enemy settlements, but the army could not afford to settle down into any lengthy sieges.

In early years this strategy worked wonders against the slow, ineffective responses from the French government. The Poitiers campaign in 1356 was the height of the chevauchées' success. By the 1370s (see the Duke of Lancaster's rather disastrous 'Great Chevauchée' in 1373) French communities had developed tactics to counter the threat posed by English raids. They knew that an English raiding army could not take heavily fortified settlements, so everyone herded their cattle and food into the nearest town and just waited. If a city or castle could stand against the English for long enough that the attackers exhausted the food sources of the surrounding countryside--which would often take only a few days or weeks at most--they would be safe as the hungry English were forced to move along to the next area.

As a side note, I am currently reading a book about another chevauchée campaign at a much later time in a much different place in history, but all the hallmarks of the Black Prince in France are there. The year is 1864 and the campaign is Sherman's March to the Sea.

Sources: wrote this off the top of my head, but it is mostly from Jonathan Sumption's fantastic Hundred Years War series, but so many others on this same topic that I've read over the years that I have lost track. If anyone is truly interested I will travel all the way from my home office to my game room bookshelf (about 30 feet) and pull down some titles.