Complex sentence: The defeat of the Athenian armament left the Segestans apparently at the mercy of their rivals; they in vain attempted to disarm the hostility of the Selinuntines by ceding without further contest the frontier district which had been the original subject of dispute.
Split into simpler sentences: The defeat of the Athenian armament apparently left the Segestans at the mercy of their rivals. They tried in vain to ease Selinuntine hostility by ceding without further contest the frontier district that was the original subject of dispute.