The Crusades, historically, are a bit of a mixed bag. On the one hand, you can trace medieval Europe’s commercial development and, eventually, its itch to explore the world in part to the East-West cultural exchange that was boosted by the Crusades, and the wealth of Arab learning that was transmitted back to Europe (including preserved texts from ancient Greece and Rome that had been lost to the Europeans) from the Middle East did jolt intellectual progress there. But in the short run they were a waste of lives and resources, and ultimately, even in the long-run, damaging to both the western (they strengthened the papacy which led to the Reformation) and eastern (the Fourth Crusade sacked Constantinople and weakened the Byzantine Empire) Christendom. They also showcase some of the most inept military leadership you’ll find in history, mostly but not entirely on the European side.
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