Complex sentence: Robert was educated at the University of Oxford, and took there a degree in civil law; since most, though not all Civil War commanders had seen military service, it is possible that he may have been a soldier for a time.
Split into simpler sentences: Robert was educated at the University of Oxford, and took there a degree in civil law. Since most, though not all of the commanders in the English Civil War had seen military service, it is possible that he may have been a soldier for a time.