When Cimon died in 449 the aristocratical party sought to counterbalance the power of Pericles by putting forward Thucydides, son of Melesias, as the new head of the party. He seems to have been an honest patriot, but, as the event proved, he was no match for Pericles. The Sacred War in 448 showed once more that Pericles knew how to defend the interests of Athens. The Phocians, under the protection of Athens, had wrested the control of the Delphic oracle from their enemies the Delphians. The latter were friendly to Sparta, and accordingly the Spartans marched into Phocis and restored the oracle to the Delphians. When they had departed, Pericles, at the head of an Athenian force, placed the oracle once more in the hands of the Phocians. As the seat of the great oracle, Delphi was to ancient Greece much what Rome was to mediaeval Europe, and the friendship of the god, or of his priests, was no small political advantage.
When the Athenians despatched a small force under Tolmides to crush a rising in Boeotia, they did so in spite of the warnings of Pericles. These warnings were soon justified by the unfortunate battle of Coronea, which deprived Athens at a blow of the continental dominion she had acquired a few years before by the battle of Oenophyta. The island of Euboea now revolted from Athens, and hardly had Pericles crossed over with an army to reduce it when word came that the Megarians had massacred the Athenian garrison, and, in league with Corinth, Sicyon, and Epidaurus, were up in arms, while a Peloponnesian army under King Plistoanax was on the point of invading Attica. Pericles recrossed in haste to Attica. The Peloponnesians returned home, having advanced no farther than Eleusis and Thria. It was said that Pericles had bribed Cleandridas; certain it is that both Cleandridas and Plistoanax were charged at Sparta with having misconducted the expedition and were found guilty. Having saved Attica, Pericles returned to Euboea, reduced it to subjection, expelled the Histiaeans, and settled the Athenian colony of Oreus on their lands.