After Thucydides' ostracism, Pericles was re-elected yearly to the generalship, the only office he ever officially occupied, although his influence was so great as to make him the de facto ruler of the state. For more than 20 years Pericles led many expeditions, mainly naval ones. Being always cautious, he never undertook of his own accord a battle involving much uncertainty and peril and he did not accede to the "vain impulses of the citizens". He based his military policy on Themistocles' principle that Athens' predominance depends on its superior naval power and believed that the Peloponnesians were near-invincible on land. (The fleet, backbone of Athenian power since the days of Themistocles, was manned almost entirely by members of the