Athens: Its Rise and Fall Book IV Vol II by Edward Bulwer-lytton

I. Cimon, summoned to the ostracism, was sentenced to its appointed term of banishment -- ten years. By his removal, the situation of Pericles became suddenly more prominent and marked, and he mingled with greater confidence and boldness in public affairs. The vigour of the new administration was soon manifest. Megara had hitherto been faithful to the Lacedaemonian alliance -- a dispute relative to the settlement of frontiers broke out between that state and Corinth. Although the Corinthian government, liberal and enlightened, was often opposed to the Spartan oligarchy, it was still essential to the interest of both those Peloponnesian states to maintain a firm general alliance, and to keep the Peloponnesian confederacy as a counterbalance to the restless ambition of the new head of the Ionian league. Sparta could not, therefore, have been slow in preferring the alliance of Corinth to that of Megara.

On the other hand, Megara, now possessed of a democratic constitution, had long since abandoned the Dorian character and habits. The situation of its territories, the nature of its institutions, alike pointed to Athens as its legitimate ally. Thus, when the war broke out between Megara and Corinth, on the side of the latter appeared Sparta, while Megara naturally sought the assistance of Athens. The Athenian government eagerly availed itself of the occasion to increase the power which Athens was now rapidly extending over Greece. If we cast our eyes along the map of Greece, we shall perceive that the occupation of Megara proffered peculiar advantages. It became at once a strong and formidable fortress against any incursions from the Peloponnesus, while its seaports of Nisaea and Pegae opened new fields, both of ambition and of commerce, alike on the Saronic and the Gulf of Corinth.