It was in the same year (432) that the great contest between Athens and Sparta, known as the Peloponnesian War, broke out. We may dismiss as a vulgar calumny the statement, often repeated in antiquity, but quite unsupported by Thucydides, that the war was brought about by Pericles for the purpose of avoiding a prosecution. The war was in truth inevitable; its real cause was Sparta’s jealousy of the growing power of Athens; its immediate occasion was the help lent by Athens to Corcyra in its war with Corinth. At first, with a hypocritical regard for religion, the Spartans demanded as a condition of peace that the Athenians should expel the race of the Alcmaeonidae (including, of course, Pericles), whose ancestors had been guilty of sacrilege about two centuries before. The Athenians retorted in kind, and, after a little more diplomatic fencing, the Spartans were constrained to show their hand by demanding bluntly that Athens should give back to the Greeks their independence—in other words, renounce her empire and abandon herself to the tender mercies of Sparta. Pericles encouraged the Athenians to reject the demand. He pointed out that Athens possessed advantages over the Peloponnesians in superior wealth and greater unity of counsels. He advised the Athenians, in case of war, not to take the field against the numerically superior forces of the Peloponnesians, but to allow the enemy to ravage Attica at will, while they confined themselves to the defence of the city. Through their fleet they would maintain communication with their island empire, procure supplies, and harass the enemy by sudden descents on his coasts. By pursuing this defensive policy without attempting to extend their empire, he predicted that they would be victorious. The people hearkened to him and replied to the Spartan ultimatum by counter-demands, which they knew would not be accepted. Pericles had not neglected in time of peace to prepare for war, and Athens was now well equipped with men, money, and ships.
In June of the following summer a Peloponnesian army invaded Attica. By the advice of Pericles the rural population, with their movables, had taken refuge in the city, while the cattle had been sent for safety to the neighbouring islands. The sight of their country ravaged under their eyes excited in the Athenians a longing to march out and meet the enemy, but in the teeth of popular clamour and obloquy Pericles steadily adhered to his defensive policy, content to protect the suburbs of Athens with cavalry. Meanwhile Athenian fleets retaliated upon the enemy’s coasts. About the same time, as a punishment for the share that they were supposed to have had in bringing on the war, the whole population of Aegina was expelled from their island to make room for Athenian colonists. This measure, directed by Pericles, relieved to some extent the pressure in the overcrowded capital, and secured a strong outpost on the side of Peloponnese. In the autumn, after the Peloponnesian army had been obliged by want of provisions to quit Attica and disband, Pericles conducted the whole available army of Athens into the territory of Megara, and laid it waste.