Complex sentence: The following day, the Spartan and Argive army returned to see who had been victorious, but each side claimed victory: the Argives because more Argives had survived and the Spartans because the Argives had retreated.
Split into simpler sentences: The following day, the Spartan and Argive army returned to see who had been victorious, but each side claimed victory. The Argives claimed victory because more Argives had survived the battle and the Spartans claimed victory because the Argives had retreated.