The same years which brought to a successful issue the long struggle with Persia witnessed a renewal of those internal conflicts by which the energies of Greece were finally exhausted, leaving her an easy prey to the arms of Macedon. The guilt of renewing these suicidal quarrels lies with the Spartans, who had long been nursing their grudge against Athens, and were waiting for the opportunity to inflict on her a fatal blow. Fifteen years [Footnote: B.C. 464. ] after the battle of Plataea they seized the occasion when the Athenians were engaged with a large part of their forces in carrying on operations against the revolted island of Thasos to prepare an invasion of Attica. But at the very moment when they were meditating this act of perfidy a double disaster fell upon them at home, demanding all their exertions to save them from ruin. Sparta was levelled to the ground by a terrible earthquake, in which twenty thousand of her citizens perished; and in the midst of the panic caused by this awful calamity the Helots rose in arms against their oppressors, and forming an alliance with the Messenian subjects of Sparta, entrenched themselves in a strong position on Mount Ithome. Here they maintained themselves for two years, defying all the efforts of the Spartans to drive them from their stronghold. In spite of their recent treachery, the Spartans were not ashamed to apply to Athens for help: and chiefly through the influence of Cimon, whose laurels from the Eurymedon were still fresh, four thousand Athenian hoplites [Footnote: Heavy-armed foot-soldiers.] were sent under his command to aid in dislodging the Helots. The Athenians were famous for their skill in attacking fortified places; but on this occasion they were unsuccessful, and the Spartans, whose evil conscience made them prone to suspicion, at once began to doubt the honesty of their intentions, and dismissed them with scant ceremony. This unfriendly act helped to embitter the relations between the two leading cities of Greece; and two years later, when the Messenians were expelled from Ithome, and driven into exile, the Athenians settled them with their families at Naupactus, an important strategic position on the north of the Corinthian Gulf, which has recently fallen into the hands of Athens. Deeply offended by the affront received at Ithome, the Athenians now formed an alliance with Argos, the ancient rival and bitter enemy of Sparta. Thessaly, connected with Athens by old ties of friendship, joined the league; and Megara, now suffering from the oppressions of Corinth, made a fourth.