During the Early Bronze Age, north-western Iran formed a single cultural zone with Armenia and southern Georgia, which entered into the orbit of what is known as the Kuro-Araxes culture. The transition from tribal-patriarchal organization to independent monarchies in both Armenia and Georgia is traditionally linked with the campaigns of Alexander the Great, and the eventual replacement of the Achaemenian empire by the much weaker Seleucid state. Under the last Persian king of the Achaemenian dynasty, Armenia enjoyed peace and prosperity. The situation in Georgia at this period was different from that prevailing in Armenia. The Romans, and later, the Byzantines, exploited their naval supremacy in the Black Sea to maintain garrisons and trading points at strategic localities in Abkhazia, Colchis and Lazistan. The adoption of Christianity by the Armenians and Georgians was to some extent a political move, designed to place the country within the orbit of Greco-Syrian civilization, and to resist cultural and religious assimilation by the Persians.