Ammianus Marcellinus was a Roman historian born around 330 AD in Syria. He is best known for his work, "Res Gestae," which covers the history of the Roman Empire from 96 to 378 AD. His writings provide valuable insights into the political and military events of his time, as well as the culture and society of the late Roman Empire. Ammianus served as a soldier before becoming a historian, which gave him a unique perspective on military affairs. His accounts are considered some of the most important sources for understanding the Roman Empire during its decline, particularly the conflicts with the Persians and the internal struggles within the empire.