The Mongol sack of Baghdad in 1258 brought an end the Abbasid caliphate in most practical senses. It was, obviously, a pivotal moment in world history, and is among the handful of events about which you can genuinely say that the world was one way before it happened and another way after it happened. At the same time, there are ways in which even such a world-changing moment was nevertheless a little anti-climactic.
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The Foreign Exchanges Companion 
I think this is one my favorite of your Middle East history posts. I always enjoy re-reading it. Fascinated to learn more about the House of Wisdom…