The battle of the Himera River (446 BC) was a clash between the Greek cities of Syracuse and Akragas, triggered by the return to Sicily of the Sicel leader Ducetius.
In 451-450 Ducetius had invaded the territory held by Akragas, and besieged Motyum. He had defeated a combined army from Syracus and Akragas at the battle of Motyum (451 BC) but in the following year was defeated at Nomae and forced into exile.
In 446 he returned to Sicily to found a new city on the north coast, at Cale Acte, or the 'fair shore' (modern Caronia). This triggered a conflict between Akragas and Syracuse. Akragas accused Syracuse of deliberately allowing Ducetius to return to Sicily. Both sides formed alliances with the other Greek cities of Sicily, and the two armies advanced to the River Himera. In ancient times there were two rivers with that name on Sicily, one that flowed north reaching the sea at the city of Himera and one that flowed south, reaching the coast at Gela, between Akragas and Syracuse. Diodorus doesn't tell us on which of these rivers the battle took place at, but the southern river would make more sense - the northern river is far to the west of Cale Acte and far distant from any route between Akragas and Syracuse.
The battle ended as a Syracusan victory. Diodorus tells us that more than a thousand Akragantini were killed in the battle. After the defeat they sent ambassadors to Syracuse, and peace was agreed. Sadly Diodorus doesn't give us any of the peace terms.