Persian Empire (reaching from Indian border to Greece border) declared war against Athens and Sparta. Athens and Sparta created coalition of small countries mostly from Central and South Greece.

Persian Army was numerically far superior (some sources giving fantastic numbers). In fact that army was so big that it had to be constantly supplied by transport ships.

Greek Coalition logically figured out that their only chance of survival was beating Persian Navy. Persian Navy had probably 650 warships (+600 transports) vs Greek coalition's 350.

 

The warships of that age needed to be beached every night. So navies needed land control of surrounding areas. Battle of Thermophylae was simply protection of Navy's flank by a land army. (intersting situation very rarely seen in history).

 

Because of the dangers of facing well-balanced, battle-hardened and very big  Persian army. Greeks has chosen to defend narrow pass of Thermophylae. There were 7000 Greek soldiers in the Greek army. Only 300 were Spartans but they were the Honor Guard of the 1 of the Spartan Kings (so very loyal and motivated). Interestingly other Spartan King was exiled and was in the Persian Emperor's entourage during this battle.

 

7000 Greeks had successfully defended the pas for several days. Pass was only 50-100 meters wide. Uncertain as the coastline has changed since that time. 1 Greek peasant then directed the Persian Army to a mountain pass ( 1 man wide at certain points). Although Greeks have known that passage, its defence was poorly handled. The elite Persian division the Immortals using that mountain pass had outflanked the Greek army. 300 Spartans and 400 Thespians sacrificed themselves to cover the retreat of other Greeks. In addition 2000 Thebai contingent, surrender to the Persian cavalry next day.

 

The more important part of the struggle Naval battle Artemisium fought at the same time period at perhaps 50 km west. By superior tactics, Greek navy successfuly defends itself, gain confidence, but could not gain any decisive results. When the land army defeated, Greek navy retreated to South.

 

Although Persian army's casualties were high should still be less than 3000+ Greek casualties.