The information available to us of events in Greece during the fifteen years which followed the battle of Plataea is limited enough. Any analysis of the relations between Athens and Sparta during this period must of necessity be a hazardous business. To explain the breakdown in the relations of the two states there is one cause so clear to us that it tends to fill the horizon, making us less sensitive to the circumstances which preceded and accompanied the rupture. The aggrandizement of Athens by her mastery of the Delian league was a factor which profoundly influenced the balance of power on the Greek mainland; but it did not, at first, bedevil Athenian–Spartan relations. There are signs of co-operation, if not of cordiality, in these relations for fifteen years; from then on there are the clearest signs of planned hostility. There is no doubt that relations deteriorated sharply at the time of the Thasian revolt, the Spartan earthquake, and the Helot revolt, and that hostility came into the open with the dismissal of the Athenian contingent at Ithome by the Spartan authorities. This action was the beginning of many misfortunes to both states: it drew aside abruptly the veil, which diplomacy had industriously maintained, from dark suspicions and sinister designs. It is the purpose of this paper to discuss the probable reasons why the break came just when it did.