Complex sentence: The Greek forces were, as agreed by the Allied congress, under the overall command of Spartan royalty in the person of Pausanias, who was the regent for Leonidas' young son, Pleistarchus, his cousin.
Split into simpler sentences: The Greek forces were, as agreed by the Allied congress, under the overall command of Spartan royalty in the person of Pausanias. Pausanias was the regent for Leonidas's young son, Pleistarchus, his cousin.