Athens and Sparta had won the Persian War together in 480-479 BC. Within twenty years they were enemies, and the long-fuse cause was a matter of walls. After Salamis, Sparta had asked the Greek cities not to rebuild their fortifications - Persia might come back, the argument ran, and walled cities made dangerous bases. Athens, suspecting another agenda, sent Themistocles to Sparta to negotiate while every Athenian who could lift a stone built. By the time Themistocles confirmed Sparta's worst suspicion, the Long Walls connecting Athens to her port at Piraeus were nearly complete. The deeper rupture came in 464 BC. Sparta had been hit by a massive earthquake and a helot revolt; the helots - enslaved Messenians who farmed Spartan land - had taken refuge on Mount Ithome. Sparta asked her allies for help. Cimon convinced Athens to send a substantial force. Sparta accepted, then turned the Athenians around at the gate, suspicious that they sympathized with the rebels. Athens took it as the insult it almost certainly was, and broke the Persian-war alliance. Cimon was ostracised.

In 458 BC, Athens began the Long Walls. A Spartan army of 1,500 hoplites and 10,000 allies marched north into central Greece, ostensibly to rescue the small region of Doris from Phocian aggression - and, less officially, to remind Athens what Spartan friendship looked like withdrawn. The job was done quickly. Then the army turned to go home and discovered that the doors were closed. The Athenian navy controlled the Corinthian Gulf. The mountain passes through the Geraneia range south to the Isthmus were held by Athenian troops based out of Megara. The Spartan general Nicomedes had a choice: try the passes and probably lose his army, or wait for the Athenians to attack him in Boeotia. He chose the second option and marched to Tanagra.

Athens sent 14,000 men under Myronides, with 1,000 allied hoplites from Argos, to meet 11,500 Peloponnesians. What happened on the field is lost. Thucydides records the result and not the maneuvers; no soldier left a description; no later historian filled in the blanks with anything reliable. Both sides claimed great losses. Sparta claimed victory and was permitted to march home through the Isthmian passes - which suggests Athens had taken the worst of it but Sparta had not made the kind of breakthrough that would let her finish the job. As the Spartans crossed the Megarid on the way back, they cut down the fruit trees of every farm they passed. It was the standard insult of Greek hoplite war: a slow harm that took a generation to undo, and announced who had won.

Sparta had won at Tanagra, gone home, and considered the matter closed. Athens did not. Sixty-two days after the battle, Myronides reformed his army and marched north again. At the Battle of Oenophyta, against a Boeotian army that no longer had Spartan reinforcement, the Athenians won decisively. The wall the Spartans had been building came down. Boeotia fell into Athenian hands. Phocis - the original pretext for Sparta's march north - was occupied. One hundred of the wealthiest citizens of Opuntian Locris were taken to Athens as hostages. Tactically, Tanagra had been a Spartan victory. Strategically, it had cost Sparta everything she had marched north to keep.

Cimon was eventually recalled from his ostracism. He had retained his peculiar credibility on both sides of the Aegean - the only Athenian both his own city and Sparta were willing to listen to - and he used it to broker a five-year peace, signed sometime in the early 450s BC, that briefly halted the First Peloponnesian War. The truce did not last; the larger war between Athens and Sparta would consume the rest of the century. But the man who had walked back from exile to fight at Tanagra and been sent away from the line went on to spend his last years trying to keep Athens and Sparta from destroying each other and the Aegean world they had built. He died on campaign in Cyprus in 450 BC. The Greeks won that battle on his name alone, after his body had already been wrapped for the trip home.