Thermopylae. The Leonidas Monument

Soon after the democracy had been established in Athens the Greek people as a whole had to undergo their most severe test. In revenge for assistance given by the Athenians and one or two other of the Greek peoples to a revolt against the Persian monarch in Ionia, across the Aegean Sea, Darius, the "Great King" of Persia, sent a naval expedition to punish the offenders. The brunt of the attack fell upon Athens. Darius, it seems, expected to be aided by the dissident antidemocratic party of Hippias, but although the latter was apparently willing to play traitor, it was unable to give him much assistance. The tactics adopted by the Persians were not well suited to the conditions, and the army which landed near Marathon in 490 b.c. was severely defeated by the Athenians, aided by the Plataeans, but without much support from any other of the Greeks. During the battle the Persians could not decide whether to use their superior navy to take Athens directly or to aid their land troops which were being beaten. This indecision meant the defeat of the entire expedition. The navy was unable to take the Piraeus, the port of Athens, and returned to Persia.

Darius bequeathed the chastisement of the Greeks to his son Xerxes, who spent the next ten years in preparing a huge if motley army which was expected to overwhelm the Greeks. In the meanwhile, however, the great Athenian leader Themistocles, well aware of the impending expedition, had persuaded the Athenians to use all their surplus money to build a fleet. But Themistocles did not have at his disposal from the citizenry of Athens a really worthwhile army. He therefore attempted to persuade the Spartans of the great danger that all the Greeks were in from the aggressive intentions of the Persians. The Spartans, however, were very jealous of the Athenians and had different notions on the strategy that ought to be employed against the Persians. Indeed, they went so far as to suggest that Greece north of the Peloponnesus was indefensible, and that a wall should be constructed to the north of the peninsula beyond which the Persians would not be able to march. Nothing had been settled when the Persian army in 480 b.c. crossed the Hellespont and proceeded into Greece from the north, receiving the submission of almost all the Greeks in their path. Too late the Spartans sent the flower of their army to stop the Persians but were overwhelmed at the battle of Thermopylae, after a traitor had betrayed to the Persians the path over the mountains by which the Spartan soldiers could be taken in the rear. The Spartans were killed to the last man, winning undying fame, but not holding up the Persians for any significant period of time.

Athens was now wide open to the invaders. By winning a great naval battle at Salamis, the Athenians prevented the Persian fleet from invading the Peloponnese. But Athens itself was captured and its citizens took refuge on the island of Salamis, just outside the Athenian harbor. The next year, for the first and almost the only time in Greek history, all the Greeks who had not submitted to the Persians joined together, and under Spartan leadership they defeated the Persians at the decisive Battle of Plataea (479 b.c). The Athenians performed their part of the bargain by again defeating the Persians on sea at the Battle of Mycale. This proved to be the end of the Persian threat until almost a century later. In the late fifth century Persia had her revenge by subsidizing and assisting the Spartans to win the Peloponnesian War, and throughout much of the fourth century it was Persian intrigues and money that kept most of the Greek states in constant enmity with one another.

After the victory the Athenians felt it to be so important to keep the Persians out of the Aegean Sea that they formed a league of the various Aegean islands and themselves, together with a few other mainland cities that agreed to join. The League, called the Confederation of Delos, had a common treasury which was maintained on the island of Delos, and each state was assessed a certain amount of either money or ships for the common cause. Although the Persians had been soundly defeated in another naval battle and there was by 460 b.c. very little apparent danger from them, Pericles insisted on keeping the League in operation, and refused to allow the city-states which wished to do so to secede. Thus the Confederation became in effect an Athenian empire. At the suggestion of one of the islands the treasury was moved to Athens, and Pericles thereafter used the money as he wished. During the next twenty years he pursued a policy of attempting to isolate Sparta and Corinth, the leading commercial city of the Peloponnese, while at the same time rebuilding Athens. Indeed, it is to the misappropriations of Pericles that we owe such wonderful buildings as the Parthenon. In defense of Pericles, it may be said that the Athenians had suffered very greatly from the common enemy and the city needed to be rebuilt after the Persian depredations. Nevertheless, the islands were not consulted on the manner in which their money was spent, and though the Athenians provided the navy in accordance with the provisions of the treaty, there was a considerable surplus of money, and Athens did, as the cities asserted, use the Confederation as its own instrument. Several of the cities of the League, indeed, complained that their freedom was being taken away, since they were not allowed to leave the League. Pericles also insisted on setting up a democratic form of government in many cities to replace the former oligarchies which he did not trust to be loyal to Athens.