The siege had its origins in a Spartan family feud. Cleonymus was a prince who believed the Agiad throne should have been his — passed over, according to Plutarch, because of his violent disposition. Adding personal humiliation to political grievance, his wife Chilonis had been seduced by Acrotatus, the son of the reigning king Areus I. Cleonymus joined Pyrrhus's army and eventually persuaded the Epirote king to march on Sparta and install him on the throne.

Pyrrhus had reasons of his own. His campaigns in Italy against Rome had been costly — victorious but ruinously so, coining the phrase "Pyrrhic victory." After returning to Epirus he had seized most of Macedonia, but the situation there was precarious. Invading the Peloponnese offered a chance to extend his power southward. He assembled an army of 25,000 infantry, 2,000 cavalry, and 24 war elephants, marched through central Greece, and entered the Peloponnese, where some of Sparta's neighbors — happy to see Spartan power diminished — offered him welcome. To the Spartan ambassadors who asked his intentions, he lied, claiming he had come to liberate cities from Macedonian control and to send his sons to be educated in Sparta's agoge.

Sparta was nearly empty of soldiers when Pyrrhus arrived. King Areus I and the main Spartan army were on campaign in Crete, supporting the polis of Gortyn. What remained of the city's defenders was a garrison force — too thin to hold the walls against an army of that size.

The Spartan gerousia, the council of elders, proposed sending the city's women to Crete for safety. Archidamia, the former queen and grandmother of King Eudamidas II, walked into the gerousia with a sword in her hand and refused. The Spartan women, she made clear, would stay and fight. What followed was remarkable: the older men and the women of Sparta spent the night before the assault digging a trench across Pyrrhus's approach. It stretched 800 feet in length, nine feet wide, and six feet deep. Wagons were sunk into the soil at the flanks to hamper the enemy's war elephants.

When Pyrrhus arrived the following morning and saw what had been built overnight by people he had not even counted as defenders, he had his answer about the character of the city he was attacking.

The Epirote assault on the first day failed to cross the trench. Pyrrhus personally led a charge but could not secure footing against Spartan resistance. His son Ptolemy tried to flank the position with 2,000 Gauls and picked Chaonians; the wagons in the soil blocked them. Acrotatus, the crown prince whose affair with Chilonis had helped start this whole war, led 300 men through a series of depressions in the ground to attack the Gauls from the rear, driving them back into the trench with heavy losses. The Spartans, watching from the walls, told Acrotatus to go rest — and return to Chilonis so he could father more sons like himself for Sparta.

On the second day, Pyrrhus himself broke through the trench on horseback, forcing his way over the wagons and into the city with a group of companions. His horse was pierced by a javelin in the belly and threw him. The fall threw his companions into confusion; the Spartans rallied and drove them back. During the night between assaults, Pyrrhus reportedly dreamed of Sparta struck by his own thunderbolts. A friend named Lysimachus warned him that places struck by lightning were avoided afterward — a bad omen for a siege. Pyrrhus dismissed the interpretation. He was wrong to.

Spartan messengers had gone out during the siege, and help was coming from two directions at once. Antigonus Gonatas — the Macedonian king whom Pyrrhus had recently driven from his own throne — dispatched his general Ameinias the Phocian from Corinth with a force of mercenaries. And Areus arrived from Crete with 2,000 men. With fresh troops on the Spartan side, the women and the elderly could finally step back from the front line, replaced by soldiers.

Pyrrhus tried one more assault on the trench. It achieved nothing. He lifted the siege.

The retreat from Laconia was a running battle. Areus pressed Pyrrhus hard, setting ambushes and seizing key positions along the route north. Pyrrhus sent his son Ptolemy to steady the rearguard. Ptolemy was killed by a Spartan war band led by a man named Evaclus. On hearing of his son's death, Pyrrhus charged the Spartans in person, killed Evaclus with his own hand, and then extracted the remnants of his army from the valley.

Pyrrhus never returned to Sparta. An invitation arrived from a faction in Argos seeking his help against a rival aligned with Antigonus, and Pyrrhus marched north toward Argolis. The expedition ended catastrophically. Entering Argos at night with part of his army, he found himself in a street fight against Argives, Macedonians, and a Spartan force under Areus all at once. In the chaos of narrow streets and close fighting, Pyrrhus was struck on the head by a roof tile thrown from above by an Argive woman — her son was in the battle below. While he lay stunned, an Antigonid soldier decapitated him.

The man who had defeated Rome twice and held all of Macedonia died in an alley, undone by a tile. His campaign in the Peloponnese, which had seemed so overwhelming at the outset, had consumed his army, cost him a son, and produced nothing. Sparta, which had neither walls nor a full garrison, had held. The alliance between Sparta and Macedon, forged in this crisis, did not survive long — but the city did.