The Turkish invasion and conquest in both the case of the Seljuks and Ottomans seems impressive. For a thousand years all these enemies like Persians, Arabs, Bulgars, Slavs, etc. couldn't bring down Byzantium but they seemed to just plow through them in the late Middle Ages. It started with Manzikert but really accelerated over the next century. And yes, I know the Crusader ordeal really hurt the Byzantines, but was that the main reason for their decline?

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Did the Turks that conquered Byzantine land in Anatolia basically quickly take the inhabitants and Turkify them, and turn them on the Byzantines themselves over a few generations? From what I understand, Turks are still mostly the native Anatolians and the Central Asian Turks were a relatively smaller elite. So basically the armies that brought down the Balkans and Constantinople eventually were mostly composed of former Byzantines, or at least descendants of them?