The eldest son of king Attalus I and queen Apollonis, Eumenes was presumably born prior to 220 BC and was the eldest of four sons to Attalus I. Eumenes followed in his father's footsteps upon becoming king and collaborated with the Romans to oppose first Macedonian, then Seleucid expansion towards the Aegean, leading to the defeat of Antiochus the Great at the Battle of Magnesia in 190 BC.