Initially the capital was established at Babylon; later at a new site at Seleuceia on the Tigris, and finally at Antioch, on the Mediterranean. The Seleucid kings pursued the easternising policy of Alexander, established Greek military and trading colonies in the east, and used Iranian manpower in their armies, but their political attention was for the most part on the west, and particularly focused on their rivalry with the other major eastern Macedonian/Greek dynasty, that of the Ptolemies in Egypt. In the east, outlying satrapies like Sogdiana and Bactria gradually became independent princedoms, the latter creating an enduring culture in what is now northern Afghanistan, fusing eastern and Greek cultures under Greek successor dynasties. Warrior Horsemen The horse-based cultures of the north-east had given Alexander problems, and the Achaemenids before him. Tribes like the Dahae and the Sakae (speaking languages in the Iranian family group), with their military strength entirely on horseback, highly mobile and able, when threatened, to disappear into the great expanses of desert and semi-desert south of the Aral Sea, would always be very difficult for any empire to dominate.
