The battle that took place in mid-480 B.C. at the narrow coastal pass called Thermopylae was fought between a small Greek force and an overwhelmingly larger force of invading Persians, and it resulted in a defeat for the Greeks defending the pass, but it also delayed the Persian advance long enough for Greek forces to regroup and ultimately repel the invaders. Greek forces were able to secure a decisive victory later that year at the Battle of Salamis. The defeat at Thermopylae created martyrs of the Greek defenders and served to increase the level of morale and determination shared by the forces defending Greece against the invaders.
After being defeated in the Battle of Salamis and fearful of being trapped in Europe, the leader of the Persian forces, King Xerxes I, withdrew most of his forces back to Asia while losing significant numbers of his men to disease and starvation. Xerxes left a portion of his forces behind to complete the invasion of the Greek Peninsula, but they were defeated by a Greek army at the Battle of Plataea the following year. The Greek victory brought a close to the Persian invasion of Greece.
The Battle of Thermopylae has often been used as an example of how a small force making good use of their training and the terrain can multiply the effectiveness of their numbers. The outnumbered Greek forces were able to hold the pass against the Persian forces for 7 days, which included 3 full days of battle before the Greeks were finally overrun.
In addition to serving as an example of defensive military strategy, both modern and ancient writers have made use of the Battle of Thermopylae as an example of soldiers displaying courage and patriotism while defending their native soil against an invader.