Having pointed out the significance of the war, not only to Greece but to the whole of the world, he gives its causes. To him the real root of the trouble was Sparta's fear of Athenian power: the alleged pretexts were different. The rise of Athens is rapidly described, her building of the walls broken down by the Persians, her control of the island-states in a Delian league which eventually became the nucleus of her Empire, her alliance with Megara, a buffer-state between herself and Corinth. This last saved her from fears of a land invasion; when she built for Megara long walls to the sea she incurred the intense anger of Corinth which smouldered for years and at last caused the Peloponnesian conflagration. The reduction of Aegina in 451 compensated for the loss of Boeotia and Egypt. Eventually the Thirty Years' Peace was concluded in 445; Athens gave up Megara, but retained Euboea; her definite policy for the future was concentration on a maritime empire; she controlled nearly all the islands of the Aegean and was mistress of the Saronic gulf, Aegina, "the eyesore of the Peiraeus", having fallen. But if she was to confine her energies to the sea, it was essential that she should be mistress of all the trade-routes which in ancient history usually ran along the coast. On both east and west she found Corinth in possession; a couple of quarrels with this city ruptured the peace. In the west, Corinth had founded Corcyra (Corfu); this daughter colony quarrelled with her mother and prevailed. In itself Corcyra was of little importance in purely Greek politics, but it happened to possess a large navy and commanded the trade-route to Sicily, whence came the corn supply. When threatened with vengeance by Corinth, she appealed to Athens, where ambassadors from Corinth also appeared. Their arguments are stated in the speeches which are so characteristic of Thucydides. The Athenians after careful consideration decided to conclude a defensive alliance with Corcyra, for they dreaded the acquisition of her navy by Corinth. But circumstances turned this into an offensive alliance, for Corinth attacked and would have won a complete victory at sea but for timely Athenian succour. In the east Athens was even more vitally concerned in trade with the Hellespont, through which her own corn passed. On this route was the powerful Corinthian city Potidaea, situate on the western prong of Chalcidice. It had joined the Athenian confederacy but had secured independence by building strong walls. When the Athenians demanded their destruction and hostages as a guarantee, the town revolted and appealed to the mother-city Corinth. A long and costly siege drained Athens of much revenue and distracted her attention; but worst of all was the final estrangement of the great trade rival whom she had thwarted in Greece itself by occupying Megara, in the west by joining Corcyra, and in the east by attacking Potidsea.