How did both domestic problems and external threats contributed to the collapse of the Persian Empire

The Empire, stretching from Libya to today’s Pakistan, coveringdisparate peoples and 4,ooo km across, in a pre-mechanicaltransport and pre-electronic age, was always difficult to control.In its attempts to exercise control, it faced continual problemsinternally and externally, there was constant unrest and invasion.While it coped with this for a couple of centuries, it was finallytoppled by the invasion of Macedonia led by Alexander the Greatwith his superior armoured infantry and methodical erosion ofPersian power on sea ad land.