A previous attempt by the Persian empire to conquer Athens had been halted at the Battle of Marathon, but the Persians continued to regard Greece as unfinished business. In 480 B.C., Xerxes, the Persian emperor, mustered a massive army of both land and naval forces and invaded Greece. Opposing the invasion was a coalition of Greek city-states headed by Athens and Sparta. The Athenian navy provided the backbone of the coalition's forces, but allied city-states provided the majority of the land troops. The Persian army and navy substantially outnumbered the defenders.
The foremost of the Greek commanders was the Athenian general Themistocles. After an attempt to bar the Persian army from entering northern Greece failed, Themistocles suggested that an infantry force stop the Persians from advancing through the pass of Thermopylae while the navy blockaded the Persian ships, preventing them from getting through the nearby Straits of Artemisium. By blocking both parts of the Persian army, Themistocles hoped to prevent the Persians from attacking the vulnerable cities of southern Greece.
The command of the allied naval forces was held by Eurybiades, a Spartan general. The largest proportion of the fleet consisted of Athenian ships, and many Athenians wanted their own general, Themistocles, to lead the allied force. However, Themistocles gave overall command to Eurybiades, acting as his adviser. The relationship between the two commanders was not completely smooth. According to the Greek historian Herodotus, Themistocles at one point bribed his Spartan counterpart to carry out his plan during the battle.
While the Persian army attempted to force its way through the pass of Thermopylae, the ships attempted to move through the Straits of Artemisium in support. A detachment of the fleet was sent to sail around the island of Euboea and surround the Greek fleet, but it was destroyed in a storm. Over the course of several days, the two fleets clashed. In the end, the battle was indecisive. When the Greek fleet received news that the Persians had finally overrun Thermopylae, they withdrew to the south.