Samos, an autonomous member of the Delian League and one of Athens' principal allies with a substantial fleet of its own, quarrels with Miletus and appeals to Athens for assistance. Pericles decides in favour of Miletus, so Samos revolts. Pericles then sails to Samos with a fleet to overthrow its oligarchic government and install a democratic one. Sparta threatens to interfere. However, at a congress of the Peloponnesian League, its members vote not to intervene on behalf of Samos against Athens.