The chief reason for this striking difference is that in the interval the Achæans, following the lead given to them by the Ætolians, had come to life and shed their ethnic cocoon. They had long since been a _koinon_, or league; but up to 251 B.C. their league, like that of Boeotia prior to 387 B.C., like that of the Ætolians prior to the seizure of Delphi in 292 B.C., had been confined strictly by the limits of their ethnos. The Ætolians had enlarged their territory under the ægis of the Delphian Amphictyony. The Achæans had no such favoring circumstance. In their case expansion by the incorporation of "foreign" peoples was the policy and achievement of a citizen of the first "foreign" city to be absorbed, Aratus of Sicyon, who saw a greater opportunity for power as the head of the neighboring league than as the "tyrant" of his native state, holding office, like the priest at Nemi, till murdered, or till he had lost the confidence of Antigonus. At his instigation the Achæan league was carried into the territory of the "foreigner," the necessary prerequisite for such a development being, however, that the ethnic bond between the Achæan cities had been canceled and replaced by a federal bond. The tenacious theory that common citizenship presupposed community of descent was therewith discarded. Its abandonment opened to the league possibilities of growth never possessed by either the city-state or the ethnic state. Of these the Ætolians and the Achæans took advantage to the best of their abilities.