Sparta fell because it could not maintain an adequate number of its warrior elite similars. They relied heavily on helots to make up the numbers and these helots went from slaves to free men mercenaries.Sparta made too many enemies with all around them and had internal bickering with non elites.Spartas enemies learnt their tactics and how to beat them over decades.Militarily the tactic sending too few men makes victory impossible and death inevitable, even if its heroic Sparta did not have an adequate economic income