Eumenes II was the eldest son of Attalus I and Queen Apollonis. Upon ascending the throne, he continued his father's policy of cooperation with Rome to counter Macedonian and Seleucid expansion.
His most significant military achievement was his support for Rome during the Roman-Seleucid War. At the Battle of Magnesia (190 BC), he personally led the cavalry charge that turned the tide against Antiochus III. The subsequent Treaty of Apamea granted Pergamon vast territories in Asia Minor, making it a major power.
Eumenes was also a brilliant cultural patron. He is credited with the substantial expansion of the Library of Pergamon, which rivaled that of Alexandria, and the construction of the Great Altar of Pergamon. Under his rule, the city became one of the most magnificent in the Hellenistic world.