War and Politics in Classical Greece: The Conflicts with the Persian Empire. Around the year 500 B.C.E. as  you know both Athenian democracy and the Spartan city-state had established themselves, but they were by no means completely stable societies, especially not in the case of Athens where Cleithenes had only recently implemented the last democratic reforms.  It was just at this time that the Greek city-states faced a great political and military challenge, maintaining their independence from the gargantuan Persian empire that was right on their doorsteps in Anatolia.  By the mid-6th century B.C.E. as Athens was moving towards its last set of democratic reforms, Persia took over the Greek-settled cities of Asia Minor (Anatolia) along its Aegean coast.  With help from the Athenian navy these cities then revolted successfully (though only temporarily so) in 499 B.C.E.  To punish Athens for her assistance to these rebellious cities the Persian emperor Darius ( 558-486 B.C.E., r. 522-486 B.C.E.) invaded Attica in 490 B.C.E., confronting the Athenian city-state and its allies at Marathon (Sparta, which had the best land-based forces, stayed home perhaps due to a rising of the helots).  The Greeks won this battle, but this was only round one, for in 480 B.C.E. Darius's successor Xerxes (~521-465 B.C.E., r. 486-465 B.C.E.) invaded Greece again, although his approach to the Greek mainland was from the north at the pass of Thermopylae.  The Spartan king Leonidas and his crack unit the Hippeis along with allies from Thebes and Thespia among other places attempted to hold this narrow and very defensible position against Darius's numerically superior forces and did so successfully until a traitor betrayed them by showing the Persians a way to outflank the Greeks' position.  This resulted in a heroic but ultimately unsuccessful last stand by the Spartans and their allies.

With this victory, the Persians found the way to Attica and to Athens open and they proceeded thence, sacking and burning the city.  But meanwhile the Athenian navy defeated the Persian navy a few months later in 480 at the Battle of Salamis and then, in 479, a combined Greek army including Spartans and Athenians defeated the Persians at the Battle of Plataea. 

War and Politics in Classical Greece: The Peloponnesian Wars. The victory over the very power Persian empire might have been an opportunity for further development of pan-Greek institutions and identity, but it did not turn out that way.  Sparta returned to its own affairs (they feared external involvements that might distract them from monitoring the helots and their revolt c. 464-460? B.C.E. would seem to bear out their caution here) and the Athenians took over sole leadership of the defensive league that was to raise money for and coordinate the ongoing efforts to defend Greece against further Persian incursions.  Known as the Delian League, this organization had its treasury on the island of Delos and  used contributions from the member  city-states to pay for collective defense.  The problem was that the Athenians were seen as and actually did make use of monies from the Delian League to support public works projects in Athens itself that did not benefit other city-states.  The Athenians also began to build up an empire with the result that the city-states of Corinth and Sparta began to worry about the ultimate end of Athenian designs. 

As a result two groups of cities, one led by Sparta the other by Athens, formed and started to fight one another.  The first conflict was c. 457-452/451 B.C.E. and is sometimes called the First Peloponnesian War.  It ended in a draw and an uneasy truce of five years and then a cold peace that lasted until 431 B.C.E. when the first half of the Peloponnesian War, the Archidamian War (431-421 B.C.E.) was fought and ended in a draw.  The Spartans won the second half of the conflict, the Decelean War (413-404 B.C.E.) and, thus, the war as a whole. 

The Consequences of the Peloponnesian Wars: The Spartan Empire and the Decline of the Greek City-States. The Spartan military leader who was decisive in Sparta's defeat of Athens was Lysander.  Lysander won a battle near Notium in 406 B.C.E.; obtained the support of the Persian Viceroy Cyrus the Younger, and was instrumental at the Battle of Aegospotami, which resulted in the destruction of the Athenian fleet in 405 B.C.E.  With the Athenian fleet destroyed, Athens could no longer secure its grain supply and it surrendered in 404 B.C.E.

Lysander then went from city-state to city-state among Athens's former allies and installed Spartan governors or harmosts with Spartan garrisons and, in some cases, Spartan-friendly ruling groups known as decarchies.  The Spartans also collected tribute from the defeated city-states.  In Athens, however, Lysander's policies resulted in the reign of the Thirty Tyrants who ruled with a heavy hand and provoked a rebellion that only the Spartan army under Lysander and the king of the Agiad House Pausanias could put down.  Pausanias compromised with the supporters of democracy and put aside the Spartan-installed rulers of Athens.  Although not everyone in Sparta approved of Pausanias's agreement with Athenian democrats, over the long run Lysander's empire could not survive even with the support of Sparta's other royal house the Eurypontid House (Agis II r. 427-399/398 B.C.E. & Agesilaus II (444-360, r. 399/398-360 B.C.E.).  Thus, by staying behind when the democratic leadership left Athens during the reign of the Thirty Tyrants, Socrates was taking a chance.  And when the democratic opposition returned to power, Socrates found himself in trouble.  The divisions within Athens after the Pelopennesian Wars should also be seen as reflective of a larger problem across all of Greek society.  The Peloponnesian Wars had exhausted the Greek city-states and by 350 B.C.E. many of them were in decline and unprepared to defend themselves against their expanding neighbor to the north, the Macedonian kingdom.