> Indeed, in the 620s the Persian shah Khusro II almost captured Constantinople and was on the verge of destroying the Roman Empire. What would have happened ha

> Indeed, in the 620s the Persian shah Khusro II almost captured Constantinople and was on the verge of destroying the Roman Empire. What would have happened had he succeeded remains one of history’s great counterfactual questions. Not really. There were a lot of people that "almost" captured Constantinople. Persians, Arabs, Bulgars, various other Steppe nomads. Constantinople due to its geography was pretty close to as impregnable as could be given ancient technology (especially when they had naval superiority). It was only the Crusaders of the 4th crusade in the 1200's and the Ottoman Turks in 1453 that successfully conquered Constantinople. The former had naval superiority thanks to the Venetians, and the latter had gunpowder. In fact, because of the strength of its defenses, one of the main strategies of the the Byzantine Empire was the tie down the bulk of the enemy's armies before the wall of Constantinople, and then attack their rear, which is exactly what the Emperor Heraclius did to the Persians leading to their defeat. In addition, the outcome of the war didn't matter so much, because it exhausted both Empires and made them ripe for being conquered by the Arab Muslim armies (the Byzantine Empire pretty much lost all their Middle Eastern provinces, and the Persian Empire was completely conquered).