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In the early 600s the Persian king Khosrau came extremely close to conquering the Byzantine Empire he failed to defeat the Byzantine Emperor Heraclius managed to defeat him and only a few years later the Persian Empire was conquered by the Arabs who were Muslim who came to rule of all the Middle East. But if Khorsau did conquer the Byzantine Empire so much would be different the Achaemenid Empire would have been revived. Islam would have never emerged and Christianity may not have been a dominant religion. Zoroastrianism may have become a dominant religion instead of Christianity and Islam.