History of the Peloponnesian War

All these actions of the Hellenes against each other and the barbarian occurred in the fifty years' interval between the retreat of Xerxes and the beginning of the present war. During this interval the Athenians succeeded in placing their empire on a firmer basis, and advanced their own home power to a very great height. The Lacedaemonians, though fully aware of it, opposed it only for a little while, but remained inactive during most of the period, being of old slow to go to war except under the pressure of necessity, and in the present instance being hampered by wars at home; until the growth of the Athenian power could be no longer ignored, and their own confederacy became the object of its encroachments. They then felt that they could endure it no longer, but that the time had come for them to throw themselves heart and soul upon the hostile power, and break it, if they could, by commencing the present war.