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Reading a bit about the Fall of Constantinople it seems to me that, that specific siege was or could not have had been such a done deal and could have had some serious butterflies.
What if the Venetian Fleet had shown up in force?
Would Giustiniani have been able to rally the defenders if he had not been killed?
What of Mehemt had been killed or wounded or if Halili Pasha had convinced the Ottoman Generals to break the siege?