In 440 BC Samos was at war with Miletus about Priene, an ancient city of Ionia on the foot-hills of Mycale.
Worsted in the war, the Milesians came to Athens with loud complaints against the Samians. Miletus was militarily weak, having been compelled to disarm and pay tribute after rebelling from Athens twice, once in the 450s and again in 446 BC; Samos, meanwhile, was one of only three remaining fully independent states in the Delian League. The Athenians, for reasons that scholars continue to disagree over—some believe that the Athenians were influenced by a desire to protect the Milesian democracy against the Samian oligarchs, while others believe that they were concerned for the credibility of their rule if they failed to protect a state that they themselves had disarmed—intervened on behalf of Miletus. A fleet of forty triremes, commanded by Pericles, was dispatched to Samos; Pericles established a democracy, and then, after taking 100 hostages to the island of Lemnos and leaving a garrison at Samos, returned home. This had all been achieved with remarkable ease, and this, in comparison with the stiff resistance that the Samians put up later, suggests that they had not expected such a harsh response from the Athenians.
The Samian War (440-439 BC) was an Ancient Greek military conflict between Athens and Samos. The war was initiated by Athens' intervention in a dispute between Samos and Miletus. When the Samians refused to break off their attacks on Miletus as ordered, the Athenians easily drove out the oligarchic government of Samos and installed a garrison in the city, but the oligarchs soon returned, with Persian support.
A larger Athenian fleet was dispatched to suppress this agitation. This fleet initially defeated the Samians and blockaded the city, but Pericles, in command, was then forced to lead a substantial portion of the fleet away upon learning that the Persian fleet was approaching from the south. Although the Persians turned back before the two fleets met, the absence of most of the Athenian fleet allowed the Samians to drive off the remaining blockaders and, for two weeks, control the sea around their island; upon Pericles' return, however, the Athenians again blockaded and besieged Samos; the city surrendered nine months later. Under the terms of the surrender, the Samians tore down their walls, gave up hostages, surrendered their fleet, and agreed to pay Athens a war indemnity over the next 26 years.