Thermoplae was a battle in around 480 BC between the persians and the spartans. it was one of the more significant and remembered battle of the persian war. In it Leonidas led a small army of spartans against a much larger army of persians. While the persians were victourious, the spartans saw this as a heroic and triumphant death. This is why the spartans are singing about it in lysistrata in another victorious moment.