The Classical period begins with one of the most dramatic military reversals in ancient history. In 490 BC, a Persian expeditionary force landed at Marathon intending to take Athens; 9,000 Athenian hoplites and 1,000 Plataeans drove them back. Ten years later, Darius's son Xerxes mounted a far larger invasion — crossing the Hellespont on a double pontoon bridge, overrunning Thrace and Thessaly, forcing the three-hundred Spartans under King Leonidas to their famous last stand at Thermopylae. Athens was evacuated and burned. But the Athenian-led naval coalition shattered the Persian fleet at Salamis in 480 BC, and the following year a Greek army defeated the Persian land force at Plataea. The victories did more than save Greece. They shattered the assumption of Persian invincibility and launched Athens into its golden age. Under Pericles, the dominant figure of the mid-5th century, the radical democracy that Cleisthenes had designed expanded further: all male citizens could attend and speak in the assembly, sit on juries, hold most offices. The city became the cultural center of the Greek world. Sophocles was staging his tragedies at the Great Dionysia. Herodotus was reading aloud from his Histories. The Parthenon was under construction. Socrates was asking his uncomfortable questions in the marketplace.

Success bred conflict. By 431 BC, Sparta and its Peloponnesian League allies had grown alarmed enough at Athenian power to go to war. The Peloponnesian War lasted twenty-seven years — longer and bloodier than any previous Greek conflict — and it reshaped the ancient world in ways that outlasted both combatants. Pericles died in a plague that killed perhaps a quarter of the Athenian population in 430–429 BC. After his death, the war party in Athens pushed for increasingly aggressive strategy. In 415 BC, the Athenian assembly voted to send a massive naval expedition to conquer Syracuse in Sicily. The general Alcibiades, its chief advocate, fled to Sparta after being recalled to face sacrilege charges. The expedition proceeded without him. It ended in total catastrophe: the entire force destroyed, the generals executed, tens of thousands of soldiers dead or captured and forced to work in the stone quarries of Syracuse under brutal conditions. Athens never fully recovered from the Sicilian disaster. Sparta, with Persian financial support, built the fleet that Athens had always dominated. In 405 BC, the Spartan admiral Lysander destroyed the Athenian fleet at Aegospotami. Athens surrendered in 404 BC, demolishing its own long walls to flute music, as the terms required.

The 4th century was a complex, sometimes bewildering succession of cities attempting to dominate Greece and failing. Sparta emerged from the Peloponnesian War as the master of the Greek world but proved temperamentally unsuited to empire. Within decades, an Athens-Corinth-Argos-Thebes coalition was challenging Spartan dominance in the Corinthian War. That war ended inconclusively in 387 BC with the Peace of Antalcidas — a humiliating settlement, essentially dictated by the Persian king, that restored Persian control over the Ionian Greeks and dissolved Greek alliances, all to prevent any single city from becoming strong enough to threaten Persian interests. Then Thebes rose. In 371 BC, the Theban general Epaminondas defeated Sparta at the Battle of Leuctra — killing a Spartan king and destroying a Spartan army — and invaded Laconia the following year. He liberated Messenia, where the helot population had been enslaved for centuries, and founded the city of Messene as their capital. Sparta never recovered the land or the labor force. But Theban dominance was equally brief. Epaminondas died at the Battle of Mantinea in 362 BC, reportedly saying that he left to Thebes 'two daughters, the victory of Leuctra and the victory at Mantinea.' After Mantinea, the historian Xenophon concluded that Greek history had become unintelligible — no city could dominate, every alliance dissolved, and the warfare continued without resolution.

Into the vacuum left by exhausted city-states came Macedonia. Philip II had inherited a kingdom on the fringe of the Greek world and transformed it into a military superpower with a professional army, a new cavalry-infantry combined-arms tactics, and the financial resources to sustain year-round campaigning. He spent the 350s and 340s extending his influence into Thrace, Thessaly, and Phocis. At the Battle of Chaeronea in 338 BC, he defeated an Athenian-Theban coalition and ended, effectively, the era of fully independent Greek poleis. He formed the League of Corinth, enlisting the Greek states into a Macedonian-led alliance with the ostensible goal of invading Persia. Philip was assassinated in 336 BC before he could launch the campaign. His son Alexander, twenty years old, inherited the plan and expanded it beyond anything his father had imagined. By 323 BC, Alexander had conquered Persia, Egypt, Bactria, and the Punjab. The Classical period was over. What followed — the Hellenistic world — was Greek in language and cultural forms but centered not on the small competitive poleis of the Aegean but on massive kingdoms ruled by Alexander's successors, stretching from Egypt to Central Asia.

The output of Classical Greece in two centuries is extraordinary by any measure. In drama: Aeschylus, Sophocles, Euripides, and Aristophanes — tragedy and comedy as art forms that still structure theatrical storytelling today. In philosophy: Socrates, Plato, and Aristotle, a chain of thinking that anchored Western metaphysics, ethics, and political theory for two millennia. In architecture: the Doric, Ionic, and Corinthian orders that recurred everywhere from Roman temples to the facades of American courthouses. In historiography: Thucydides' analysis of the Peloponnesian War, which introduced the concept of studying history to understand causation rather than simply to record events. In medicine: Hippocrates of Cos and his school, who insisted that disease had natural explanations and established clinical observation as a medical method. Much of this was produced in a city of perhaps 250,000 people — including a very large enslaved population — in an era when Athens was simultaneously fighting for survival. The Classical achievement was not despite the violence and instability of the period; in some difficult-to-explain way, it emerged from that very condition of permanent crisis and competition.