The Persian defeat at Marathon in 490 B.C. did not spell the end of Persian aggression, as was proved by a new invasion, that of Xerxes in 480 B.C. Yet, for the Hellenes, their survival against the superior forces of the barbarian was of legendary significance. It became the symbol of the triumph of a democratic city, and in the course of the 5th century was promoted as the point of origin of Athenian might. The reinforcement of the authority of the generals after the battle led to a replacement in the way archons were chosen - no longer by election but by lot, from among the pentakosiomedimnoi and hippeis. Rivalry between the aristocratic families continued even after the invasion, and for the first time the measure known as ostracism was put into force. Those who were now banished, on Themistocles' instigation, as dangerous to democracy were Hipparchus the Pisistratid, Aristides, Xanthippus father of Pericles, and Megacles the Alcmaeonid.

Themistocles was the first to perceive the importance of shipbuilding for a fleet capable not only of fending off any future invasion but of securing the city's food supply. Athens' defeat at Paros may have contribuited to the adoption of his proposal by the Athenian demos. In 483/2 B.C. a new lode of silver was discovered at the Laurium mines, and this brought the city one hundred talents. Themistocles opposed Aristides' proposal that the money should be shared out among the demos, and made sure that it was given to a hundred well-to-do cities with the obligation that each should fit out one trireme. The timber was supplied by Alexander I of Macedon: though a tributary of Persia, he had not broken off relations with Athens. The new fleet was manned by members of the lowest class, that of the thetes, who - in the context of the Athenian body politic - thus increased their specific gravity relatively to the hoplites. This may have been the reason for Aristides' opposition, the consequence of which was his ostracism. By building a fleet, Athens paved the way for her showdown with a great power, thus counterpoising Spartan superiority in land power, and ensured her survival in face of the Persians.