How was the development of the Athenian Empire a product of the successful repulsion of the Persian invasions

After seeing off the Persian threat, the Greek cities were able toreturn to their habitual shifting alliancces and disputes. Thediffrence this time was that Athens, had organised the anti-Persiandefensive Delian League to ensure the cities of Asia Minor and theIslands were free from the Persian threat. When peace was finallyformalised after 449 BCE Athens shifted this Delian League treasuryfrom the island of Delos to Athens and then began to spend it onitself, building the Parthenon etc and putting nearly half itspopulation on the payroll, and paying for its fleet to continuecollecting League contributions by force if necessary.

This League therefore became effectively an empire of Athens, andAthens was thus enabled to use its power to intervene in othercities’ affairs outside its empire. Sparta was prevailed on to leada Peloponnesian League as a counter (most were Dorian cities inpeninsular Greece – the Peloponnese). So the threat of Persia wasreplaced with threat of war between two powerful internal Greekcoalitions, which came devastatingly into play with outbreak of thePeloponnesian War in 431 BCE. Paradoxically, this war was decidedagainst Athens after 27 years by the late intervention of Persia onthe side of the Peloponnesians.