With the help of these allies, Chalcis engaged the rival league of its neighbor Eretria in the so-called Lelantine War, by which it acquired the best agricultural district of Euboea and became the chief city of the island. Late in the 6th century BC, its prosperity was broken by a disastrous war with the Athenians, who expelled the ruling aristocracy and settled a cleruchy on the site. Chalcis subsequently became a member of both the Delian Leagues.