Eumenes of Pergamon had in 263 BCE succeeded his uncle Philetaerus (reportedly a eunuch), who had abandoned the Seleucids in favor of a tie with Egypt, and defeats the Seleucid army near Sardis (the old capital of Lydia) in 262 BCE, thereby establishing Pergamon as an independent city-state.

Pergamon has great resources in silver, agriculture, and stockbreeding, but only now comes to marked prominence.

Pergamon, originally a mountain fortress on the Aegean coast, has become an important continental power through the careful maneuvering of its rulers, Philetaerus (282-263) and later his nephew Eumenes (263-241).

Attalus, whose mother, Antiochis, is a princess of the Seleucid house, in 241 succeeds his uncle Eumenes.