In the history of the ancient world, Bactria is somewhat of an anomaly: a Greek kingdom in what is now Afghanistan. That Greek civilization penetrated so far into Central Asia is quite astounding in itself. When Alexander the Great conquered the Persian Empire, he acquired all its outlying provinces including Bactria. Greek forces then established and maintained control in Bactria even after the collapse of the Alexander’s Kingdom. Bactria was at first part of the eastern section of Alexander's Kingdom, which was ruled by the Seleucids. There was extensive immigration of Greeks and the creation of Greek cities. These cities were built on the Greek model and included such pillars of Greek culture as gymnasiums and amphitheatres. Later Bactria asserted its independence and expanded its holdings to the upper reaches of the Indus River Valley. The Greek State in Bactria lasted for another two centuries, until it was finally overwhelmed by the nomadic tribesmen of the area and was eventually absorbed into the Kushan Empire.