The Spartans were given command of the united Greek forces during the time of the Persian wars as they were seen as the more powerful military force in Greece. This was the first and last time throughout Greek history that would see the entire Greek world united (Brunt 1953). The Spartans upon hearing word that the Persians were advancing again decided against sending their entire army as it would’ve been seen as sacrilegious due to the Olympic festival being held at the same time. They realised that doing nothing would not be acceptable so they sent a small expedition led by Leonidas one of its kings, to block the pass until the festival was over and the main army could arrive to back them up. According to Herodotus the army led by Leonidas was also sent in the hopes to rally several of the other allied states into joining them in the defence. Leonidas himself believed that due to a prophecy the Spartans had received from the Oracle of Delphi (Rawlinson 1942 [7.220]) that he was going to die in this battle and thus was prepared to do so from the start. The courage and honor shown by the Spartans and its king leading up to the battle are worthy of mention, but the battle that followed is also an important part of the celebration in itself.
Xerxes had arrived at Thermopylae with a force that outnumbered the Greeks by many tens of thousands. He expected very little resistance and after receiving word about the Greeks that had made
References: P.A. Brunt, ‘The Hellenic League against Persia’, in Historia 2 (1953), pp.135-62 M.A. Flower, ‘Simonides, Ephorus and Diodorus on the battle of Thermopylae’, in CQ 48 (1998), pp.365-79 R. Hope-Simpson, ‘Leonidas’ decision’, in Phoenix 26 (1972), pp.1-1 G. Rawlinson, ‘The Persian Wars by Herodotus’ (1942), Book IIV