1) The Death of Alexander the Great: It is commonly said that the Death of Alexander the Great was the death of his empire. As he had no legitimate son to take his place, there was no lineage in which to pass down his power. This led to the second point.
2) The power grab between his generals: The generals under him instinctively divided up his empire, but with greed, came war, and they typically fought with each other in an effort to take over as much territory for themselves as possible. This led to a general weakening of his once great empire, and the destruction of it.
