Complex sentence: Heraclius' defeat of the Persians had been the end game in a war that had been on and off for almost 800 years, when Alexander the Great had totally defeated the Persians.
Split into simpler sentences: Heraclius' defeat of the Persians had been the end game in a war that had been on and off for almost 800 years. It was then that Alexander the Great had totally defeated the Persians.