Alexander the Great's Legacy

Why was it that 2400 years ago the people who had recently conquered the world were unable to stop barbarian Galatians from looting the tombs of their revered royal line? Why was it that the Macedonian state virtually created by Philip II and taken to the heights of epochal triumph by his son Alexander the great had, hardly two generations after his death , became a weaker entity than it had been when the young conqueror had crossed the Hellespont? This was a period during which Cassander and Ly