Leonidas's name meant "lion-like" and he was one of three sons of King Anaxandridas II. He became one of two Spartan kings, occupying the throne at the same time in a tense but civil arrangement.
In 480 BC King Xerxes of Persia lead a massive army against the notoriously fractious city-states of Greece. In part it was to avenge his father, the great Darius, whose stinging defeat at Marathon, some ten years earlier, had saved the free world. In part Xerxes wanted to attain one of his father's goals, an even larger Persian empire.
Ancient reports put Xerxes' forces at 1.5 to 2 million people, and with attendants, followers and courtesans, it was stated to have been five million. More modern estimates put it at approaching 500,000.