Complex sentence: Roman historians (like Ptolemeus) considered that all actual Morocco until the Atlas mountains was part of the Roman Empire, because in the Augustus times Mauretania was a vassal state and his rulers (like Juba II) controlled all the areas south of Volubilis.
Split into simpler sentences: Roman historians (like Ptolemeus) considered that all actual Morocco until the Atlas mountains was part of the Roman Empire. During the time of Augustus, Mauretania was a vassal state and his rulers (like Juba II) controlled all the areas south of Volubilis.