Complex sentence: In his first battle, at Mantinea, Philopoemen defeated Machanidas, the Spartan king, and, then, he exhibited his severed head, throughout the Peloponnesus, whose nations would surrender eventually, to his feared Achaean army.
Split into simpler sentences: In his first battle, at Mantinea, Philopoemen defeated Machanidas, the Spartan king. Subsequently, the Peloponnesian nations would surrender eventually, to his feared Achaean army.