What message did the conquest of the byzantine empire send to the rest of europe ?

The conquest of the Byzantine Empire sent the rest of Europe a message that a great force is coming from the east and that it's territories, culture, religion, are all under danger of being overtaken and assimilated into another territory, culture, religion. That lead to unification of the European countries/empires in the sense of having a same enemy, so they helped each other to stop the progress of the upcoming eastern force that were the Ottomans.

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