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. Pericles also tried to minimize the advantages of Sparta by rebuilding the walls of Athens, which, it has been suggested, radically altered the use of force in Greek international relations.