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

Answer 1

After seeing off the Persian threat, the Greek cities were able to return to their habitual shifting alliancces and disputes. The diffrence this time was that Athens, had organised the anti-Persian defensive Delian League to ensure the cities of Asia Minor and the Islands were free from the Persian threat. When peace was finally formalised after 449 BCE Athens shifted this Delian League treasury from the island of Delos to Athens and then began to spend it on itself, building the Parthenon etc and putting nearly half its population on the payroll, and paying for its fleet to continue collecting League contributions by force if necessary.

This League therefore became effectively an empire of Athens, and Athens was thus enabled to use its power to intervene in other cities' affairs outside its empire. Sparta was prevailed on to lead a Peloponnesian League as a counter (most were Dorian cities in peninsular Greece - the Peloponnese). So the threat of Persia was replaced with threat of war between two powerful internal Greek coalitions, which came devastatingly into play with outbreak of the Peloponnesian War in 431 BCE. Paradoxically, this war was decided against Athens after 27 years by the late intervention of Persia on the side of the Peloponnesians.