control of vast territories to Athens. Athens had greatly increased its own power; a number of its formerly independent allies were reduced, over the course of the century, to the status of tribute-paying subject states of the Delian League. This tribute was used to fund a powerful fleet and, after the middle of the century, massive public works in Athens, causing resentment. Friction between Athens and the Peloponnesian states, including Sparta, began early in the Pentecontaetia. In the wake of the departure of the Persians from Greece, Sparta sent ambassadors to persuade Athens not to reconstruct their walls, but was rebuffed. Without the walls, Athens would have been defenseless against a land attack and subject to Spartan control. According to Thucydides, although the Spartans took no action then, they "secretly felt aggrieved". Conflict between the states flared up again in 465 BC, when a helot revolt broke out in Sparta. The Spartans summoned forces from all of their allies, including Athens, to help them suppress the revolt. Athens sent out a sizable contingent (4,000 hoplites), but upon its arrival, this force was dismissed by the Spartans, while those of all the other allies were permitted to remain. According to Thucydides, the Spartans did this out of fear that the Athenians would switch sides and support the helots; the offended Athenians repudiated their alliance with Sparta. When the rebellious helots were finally forced to surrender and permitted to evacuate the state, the Athenians settled them at the strategic city of Naupaktos on the Gulf of Corinth. In 459 BC, there was a war between Spartan allies Megara and Corinth, which were neighbors of Athens. Athens took advantage of the war to make an alliance with Megara, giving Athens a critical foothold on the Isthmus of Corinth. A 15-year conflict, commonly known as the First Peloponnesian War, ensued, in which Athens fought intermittently against Sparta, Corinth, Aegina, and a number of other states. For a time during this conflict, Athens controlled not only Megara but also Boeotia. But at its end, a massive Spartan invasion of Attica forced Athens to cede the lands it had won on the Greek mainland, and Athens and Sparta recognized each other's right to control their respective alliance systems. The war was officially ended by the Thirty Years' Peace, signed in the winter of 446/5 BC. Breakdown of peace The Thirty Years' Peace was first tested in