Thermopylae was important because Persia was the world's first true superpower. It dominated Asia and sought to dominate Europe. If the Greeks had lost at Marathon, Western civilization would have died at birth. And the Greeks could not have won at Marathon without the brave stand of the Spartans at Thermopylae.
To call what the Spartans did at Thermopylae 'brave' is to misunderstand the scale of the action. Thousands of Persian soldiers, including the very best, the 'Immortals', tried repeatedly to destroy the tiny force that blocked their way.
Every Greek soldier was outnumbered more than ten to one in each clash. They knew, with an absolute certainty, that not one of them would survive the battle, and yet they kept the mighty Persian army at bay for a
critical six days.
Leonidas and his men knew what was at stake, and knew that every hour they bought for the army assembling at Marathon was just that much more chance that the Greeks could defeat the Persian juggernaut before them. Many men have risked death for their countries, but these men took no risk: death was a certainty. Yet even though the way to flee was clear behind them, they held the pass to the last man,
even launching a series of daring charges to recover the body of Leonidas once he had died, for he stood
in the front rank throughout the fighting.