In the Peloponnesian War the Boeotians fought zealously against Athens. Although slightly estranged from Sparta after the peace of Nicias, they never abated their enmity against their neighbours. They rendered good service at Syracuse and at the Battle of Arginusae in the closing years of the Pelopennesian War; but their greatest achievement was the decisive victory at the Battle of Delium over the Athenian army (424 BC) in which both their heavy infantry and their cavalry displayed unusual efficiency.