431 BC

Peloponnesian War

The Theban night attack on Plataea opens the Peloponnesian War

A city gate opened from the inside on a spring night, admitting just over 300 armed men while the Thirty Years' Peace technically still held.

Spring 431 BCE: the gate opens

At the beginning of spring in 431 BCE, slightly more than 300 armed Thebans entered Plataea — a Boeotian city allied with Athens — at night. Naucleides and fellow Plataean conspirators opened the gates for them.

The intention was to kill their political opponents and transfer the city to the Theban side. The Boeotarchs Pythangelus and Diemporus commanded the force.

A hundred and eighty prisoners

The invaders tried persuasion rather than immediate slaughter, which gave the Plataeans time to work out how few of them there were. Most of the city then attacked, killing some Thebans and capturing 180.

After the supporting Theban force withdrew, the Plataeans executed the prisoners. Athens' order to wait for instructions arrived too late.

Where the war begins depends on the passage

Thucydides explicitly marks this episode as the point at which war between the two alliances actually began — not Archidamus's invasion of Attica, which came roughly 80 days later.

Elsewhere he emphasizes that invasion instead, so modern scholarship distinguishes the first violence from Sparta's formal opening campaign. Potidaea, Corcyra and the Megarian Decree were antecedent disputes, not this night's event.

Athens seized Boeotians who were in Attica, then provisioned and garrisoned Plataea and removed its vulnerable residents. The Spartan siege of the city is a separate event, beginning in 429 BCE.

Sources

Researched 23 Aug 2026 6 sources 2 audit passes

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What the sources leave uncertain

The year 431 BCE is secure, but proposed conversions of Thucydides' seasonal and civic-calendar dating into an exact modern day vary.
Thucydides sometimes frames the Theban attack on Plataea as the beginning and elsewhere emphasizes Sparta's invasion of Attica roughly 80 days later; modern scholarship therefore distinguishes the first violence from Sparta's formal opening campaign.