Why was the Persian Empire weakning

Alexander the Great first captured its Mediterranean ports so thatit could no longer pose a naval threat. He massacred the Greekmercenaries in Persian service after the battle of the Granicus sothat Greeks would no longer provide the armoured infantry thePersians needed, and he captured the Persian treasury after thebattle of Issus. These measures stripprd the Persians of theirunderlying strengths.