"e;The Feudal Loop - How a leather strap created the knight and destroyed empires"e; argues that the entire social structure of the Middle Ages-knights, castles, and feudalism-rested on a single, small invention: the stirrup. Before the stirrup, riders dangled their legs and could not strike with force without falling off. With the stirrup, horse and rider became one solid projectile. Historian Julian West traces the migration of this technology from the Asian steppes to Europe, where it allowed Charles Martel to create a new class of "e;shock cavalry"e; to defeat the Arab invasion at Poitiers in 732. The book explains the economic consequence: armoring and training these mounted warriors was so expensive that land had to be granted to them in exchange for service, creating the feudal system. "e;The Feudal Loop"e; is a micro-history of unintended consequences. It shows how a simple piece of leather and iron locked Europe into a social hierarchy that lasted for almost a thousand years.