History of the Peloponnesian War

For if the command of the sea and land were united in one hand, he would not know where to turn for help to overthrow the dominant power; unless he at last chose to stand up himself, and go through with the struggle at great expense and hazard. The cheapest plan was to let the Hellenes wear each other out, at a small share of the expense and without risk to himself.

Besides, he would find the Athenians the most convenient partners in empire as they did not aim at conquests on shore, and carried on the war upon principles and with a practice most advantageous to the king; being prepared to combine to conquer the sea for Athens, and for the king all the Hellenes inhabiting his country, whom the Peloponnesians, on the contrary, had come to liberate. Now it was not likely that the Lacedaemonians would free the Hellenes from the Hellenic Athenians, without freeing them also from the barbarian Mede, unless overthrown by him in the meanwhile.