The Peloponnesian War started with a covert operation. It was important for Thebes to capture Plataea, because it controlled the roads to the Theban allies on the Peloponnese (Corinth and Sparta), and it is not surprising that the Thebans wanted to capture the town. They preferred to take Plataea by treason: a traitor let them in during the rainy night of c. 6 March 431. It was a complete violation of the code of honor that had once ruled Greek warfare, but, as Thucydides pointed out in a different context, "The ancient simplicity was laughed down and disappeared".note[Thucydides, History of the Peloponnesian War 3.83.1.]
During the night, the traitors discovered that the Thebans, who were supposed to kill many Plataeans, had completely different designs. As could be expected, those Plataeans who had been surprised by the conspiracy fought back, and in the end captured the Theban attackers. They were almost immediately killed. Violation of a peace treaty, treason, double-crossing, killing POWs: the attack on Plataea was a worthy prologue to the war.
The translation of History of the Peloponnesian War 2.2.1-6.3 was made by Richard Crawley.
Thucydides on the outbreak of the Archidamian war
[2.2.1] The Thirty Years' truce,note[A treaty that had been concluded in 446/445 and marked the end of an earlier war between Athens and Sparta.] which was entered into after the conquest of Euboea, lasted fourteen years. In the fifteenth, in the forty-eighth year of the priestessship of Chrysis at Argos, in the ephorate of Aenesias at Sparta, in the last month but two of the archonship of Pythodorus at Athens, and six months after the battle of Potidaea, just at the beginning of spring, a Theban force a little over 300 strong, under the command of their Boeotarchs, Pythangelus, son of Phyleides, and Diemporus, son of Onetorides, about the first watch of the night, made an armed entry into Plataea, a town of Boeotia in alliance with Athens.
[2.2.2] The gates were opened to them by a Plataean called Naucleides, who, with his party, had invited them in, meaning to put to death the citizens of the opposite party, bring over the city to Thebes, and thus obtain power for themselves.
[2.2.3] This was arranged through Eurymachus, son of Leontiades, a person of great influence at Thebes.
Plataea had always been at variance with Thebes; and the latter, foreseeing that war was at hand, wished to surprise her old enemy in time of peace, before hostilities had actually broken out. Indeed this was how they got in so easily without being observed, as no guard had been posted.
[2.2.4] After the soldiers had grounded arms in the market-place, those who had invited them in wished them to set to work at once and go to their enemies' houses. This, however, the Thebans refused to do, but determined to make a conciliatory proclamation, and if possible to come to a friendly understanding with the citizens. Their herald accordingly invited any who wished to resume their old place in the confederacy of their countrymennote[Thebes was the largest city in Boeotia and leader of the Boeotian federation. The Thebans offered Plataea membership of this alliance.] to ground arms with them, for they thought that in this way the city would readily join them.
[2.3.1] On becoming aware of the presence of the Thebans within their gates, and of the sudden occupation of the town, the Plataeans concluded in their alarm that more had entered than was really the case, the night preventing their seeing them. They accordingly came to terms and, accepting the proposal, made no movement; especially as the Thebans offered none of them any violence.
[2.3.2] But somehow or other, during the negotiations, they discovered the scanty numbers of the Thebans, and decided that they could easily attack and overpower them; the mass of the Plataeans being averse to revolting from Athens.
[2.3.3] At all events they resolved to attempt it. Digging through the party walls of the houses, they thus managed to join each other without being seen going through the streets, in which they placed wagons without the beasts in them, to serve as a barricade, and arranged everything else as seemed convenient for the occasion.
[2.3.4] When everything had been done that circumstances permitted, they watched their opportunity and went out of their houses against the enemy. It was still night, though daybreak was at hand: in daylight it was thought that their attack would be met by men full of courage and on equal terms with their assailants, while in darkness it would fall upon panic-stricken troops, who would also be at a disadvantage from their enemy's knowledge of the locality. So they made their assault at once, and came to close quarters as quickly as they could.
[2.4.1] The Thebans, finding themselves outwitted, immediately closed up to repel all attacks made upon them. Twice or thrice they beat back their assailants.
[2.4.2] But the men shouted and charged them, the women and slaves screamed and yelled from the houses and pelted them with stones and tiles; besides, it had been raining hard all night; and so at last their courage gave way, and they turned and fled through the town. Most of the fugitives were quite ignorant of the right ways out, and this, with the mud, and the darkness caused by the moon being in her last quarter, and the fact that their pursuers knew their way about and could easily stop their escape, proved fatal to many.
[2.4.3] The only gate open was the one by which they had entered, and this was shut by one of the Plataeans driving the spike of a javelin into the bar instead of the bolt; so that even here there was no longer any means of exit.
[2.4.4] They were now chased all over the town. Some got on the wall and threw themselves over, in most cases with a fatal result. One party managed to find a deserted gate, and obtaining an axe from a woman, cut through the bar; but as they were soon observed only a few succeeded in getting out. Others were cut off in detail in different parts of the city.
[2.4.5] The most numerous and compact body rushed into a large building next to the city wall: the doors on the side of the street happened to be open, and the Thebans fancied that they were the gates of the town, and that there was a passage right through to the outside.
[2.4.6] The Plataeans, seeing their enemies in a trap, now consulted whether they should set fire to the building and burn them just as they were, or whether there was anything else that they could do with them;
[2.4.7] until at length these and the rest of the Theban survivors found wandering about the town agreed to an unconditional surrender of themselves and their arms to the Plataeans.
[2.4.8] While such was the fate of the party in Plataea
[2.5.1] the rest of the Thebans who were to have joined them with all their forces before daybreak, in case of anything miscarrying with the body that had entered, received the news of the affair on the road, and pressed forward to their succor.
[2.5.2] Now Plataea is nearly thirteen kilometer from Thebes, and their march delayed by the rain that had fallen in the night, for the river Asopus had risen and was not easy of passage;
[2.5.3] and so, having to march in the rain, and being hindered in crossing the river, they arrived too late, and found the whole party either slain or captive.
[2.5.4] When they learned what had happened, they at once formed a design against the Plataeans outside the city. As the attack had been made in time of peace, and was perfectly unexpected, there were of course men and stock in the fields; and the Thebans wished if possible to have some prisoners to exchange against their countrymen in the town, should any chance to have been taken alive.
[2.5.5] Such was their plan.
But the Plataeans suspected their intention almost before it was formed, and becoming alarmed for their fellow citizens outside the town, sent a herald to the Thebans, reproaching them for their unscrupulous attempt to seize their city in time of peace, and warning them against any outrage on those outside. Should the warning be disregarded, they threatened to put to death the men they had in their hands, but added that, on the Thebans retiring from their territory, they would surrender the prisoners to their friends.
[2.5.6] This is the Theban account of the matter, and they say that they had an oath given them. The Plataeans, on the other hand, do not admit any promise of an immediate surrender, but make it contingent upon subsequent negotiation: the oath they deny altogether.
[2.5.7] Be this as it may, upon the Thebans retiring from their territory without committing any injury, the Plataeans hastily got in whatever they had in the country and immediately put the men to death. The prisoners were a 180 in number; Eurymachus, the person with whom the traitors had negotiated, being one.
[2.6.1] This done, the Plataeans sent a messenger to Athens, gave back the dead to the Thebans under a truce, and arranged things in the city as seemed best to meet the present emergency.
[2.6.2] The Athenians meanwhile, having had word of the affair sent them immediately after its occurrence, had instantly seized all the Boeotians in Attica, and sent a herald to the Plataeans to forbid their proceeding to extremities with their Theban prisoners without instructions from Athens.
[2.6.3] The news of the men's death had of course not arrived; the first messenger having left Plataea just when the Thebans entered it, the second just after their defeat and capture; so there was no later news. Thus the Athenians sent orders in ignorance of the facts; and the herald on his arrival found the men slain.
[2.6.4] After this the Athenians marched to Plataea and brought in provisions, and left a garrison in the place, also taking away the women and children and such of the men as were least efficient.