Modern people think of an "arms race" as a competition between superpowers to see who can create the biggest and deadliest weapons, but the Middle Ages had an arms race of its own. Mark Teppo of Wired Magazine looks at weapons escalation in the medieval world.
Teppo, author of the Mongoliad book series, has long been interested in knights and swords, but in researching his books, he "began a crash course in the evolution of arms and armor over several centuries of medieval life," from maille armor to cavalry, to two-handed swords, etc.
Suddenly, the knight’s kit started to include the great helm and chausses, maille leggings attached to the waist with straps under the long skirt of the hauberk. The gear tweakers and data miners kept comparing notes, and other bits of metal started showing up: spaulders (protecting the shoulders), greaves (covering the shins), gorgets (keeping the throat safe from thrusts), gauntlets and vambraces (covering the elbows and forearms). All these pieces of plate were meant to deter the slashing and cutting attack of the longsword, and in response to this gradual turtle-shelling, the swords changed, too.