Selinus and the nearby non-Greek (Elymian) city of Segesta were enemies; Segesta persuaded Athens to come to their aid in 415 B.C. by fooling the Athenians into thinking they had a lot more money available than they did--the beginning of the disastrous Sicilian Expedition of 415-413 which was an important contributing factor in the eventual Athenian defeat in the Peloponnesian War.
Segesta’s ruins look pretty nice as well; as a consequence of that war, the town ran out of cash and its rather ambitious civic construction program stopped dead, leaving it unfinished (if memory serves). Thanks VR!