A major Hellenistic state in Western Asia that existed from 312 BC to 63 BC. Founded by Seleucus I Nicator, it controlled vast territories including Anatolia, Persia, the Levant, Mesopotamia, and parts of India at its height. Its capitals were Seleucia on the Tigris and later Antioch on the Orontes. The capital was moved to Antioch to be closer to the Mediterranean and Greece, ensuring better access to mercenaries and trade, and to more effectively manage conflicts with western rivals.