On a dark night, 300 Theban warrior crept up on Platea and were admitted by the local oligarchic party. The oligarchs wanted the Thebans to kill the democratic leaders and seize the city by force. But the Thebans, with that oligarchic denseness, believed they could win the Plateans over. At first the Plateans wer intimidated and negotiated, but once they realized that it was only a small force in the city, they resolved to resist, not wanting to break the alliance with Athens. The Plateans attacked. Women climbed up on rooftops and dropped tiles on the Thebans. Some Thebans were killed, some escaped, and the greater number opened a door they thought was the city gates and stumbled into a building with no way out, so they surrendered. The larger Theban force was delayed by rain and flooding and turned back when the Plateans threatened to kill the captured force, numbering about 180. Once the Thebans withdraw, the Plateans executed the captured soldiers anyhow. They then sent for help from Athens. The Athenians warned the Plateans not to harm their prisoners, but it was too late. So the best they could do was evacuate the non-combatants and leave a small force to help dig in for a siege. Only a garrison of 400 Platean men, reinforced by 80 Athenians, and with 110 women in support roles remained.
It was two years later, in 429, that the Spartans came to the aid of Thebes. When the Plateans appealed to their alliance in fighting the Persians, the Spartans demanded that the Plateans either remain neutral or leave their city, which the Spartans promised to return to them after the war. They argued that they were trying to "free" Platea, and the rest of Greece, from Athenian domination. The Plateans consulted with Athens and foolishly accepted their assurances of protection, and therefore stuck with the alliance. The Spartan king then effectively said that they were not the aggressors, and that the Plateans had brought their fate upon themselves by taking the wrong side in the war. A siege ensued. It continued for two years. No aid arrived from Athens.
About half the force managed to escape and make it back to Athens. By 427 B.C., the Plateans' supplies had run out and no relief was arriving from Athens, so they surrendered, on the promise that they would be treated fairly. Instead, they were accused of the "crime" of having fought against Sparta in the present war. Of course, since Platea was an Athenian ally, it was completely proper for the Plateans to oppose Sparta in the war. But the Thebans argued that siding with the Athenians was in itself a crime to be punished. The Thebans also defended their invasion of Platea in peace time on the grounds that they were invited in by "the noblest and richest of your citizens."
Like yourselves they were citizens, and they had a greater stake in the country than you have; they opened their own gates and received us into their native city, not as her enemies but as her friends. They desired that the bad among you should not grow worse, and that the good should have their reward. They wanted to reform the principles of your citizens, and not to banish their persons.The Thebans were angry that the Plateans attacked the invading party, and angrier still (and more justifiably) that they executed them after they had surrendered. So the Spartans executed the men for the "crime" of having fought against then, sold the women as slaves, and destroyed the city. They justified their actions by saying that they gave the Plateans the chance to be neutral or withdraw, but they refused to take it. Thucydides attributes the Spartans' severity to their desire to stay on the good side of the Thebans. Obviously we have no way of knowing which of these self-justifying rationalizations the Thebans and Spartans actually believed. But the Thebans do seem remarkably dense in their refusal to recognize why the Plateans might want to preserve their democracy from the "noblest and richest" of their citizens,or why they might see Thebes as a greater threat to their sovereignty than Athens.
It may seem unfair to include tiny Platea, overwhelmed by more powerful armies among the failures of democracy. To call Platea a failure of democracy because it was conquered by Thebes is like calling Belgium, the Netherlands, Denmark and Norway failed democracies because they were overrun by Hitler. But Platea succumbed, not just to Thebes, but to traitors within. And it was the democracy that made the ultimately suicidal decision of defiance. And Platea illustrates very well the point I made in my last post about how dense oligarchs could be in failing to understand the appeal of Athens to local democrats.