The Battle of Thermopylae: 300 Spartans vs. A Million Persians - Who Will Prevail?

In 499 BC Greek cities which had been captured by the Persians in Asia Minor revolted against the brutal tyrants that had been placed to oversee them. In support of their conquered brethren, Athens and Eretria sent troops. Despite some major gains, several strategic mistakes cost the Greeks of Asia Minor their ultimate victory and the rebellion was put down.

With Asia Minor back in the fold of the Persian Empire, the Persian king Darius I vowed to punish Athens and Eretria for their involvement, and saw the rest of the free cities of Greece as a threat to his empire. In 492 BC he launched an invasion of Thrace and Macedon, then sent heralds to the remaining Greek city-states demanding they accept Persian rule. Seeking to save themselves, many agreed- with the notable exceptions of Athens and Sparta.
The Persian heralds in Athens were thrown into a pit, and their Spartan brethren followed suit by tossing theirs into a well. Enraged, Darius launched his invasion of mainland Greece and met with further success until an encounter against 10,000 Athenians in Marathon. Outnumbering the Greeks by 2.5 to 1, Darius saw an easy win- only for the Athenians to achieve a dramatic victory and force Darius to retreat.

Nursing a very wounded ego, Darius planned an imminent re-invasion, with plans to raze Athens to the ground- but internal politics delayed these plans and Darius died of old age. Seeking to avenge the pride of his dead father, Xerxes prepared for a decisive campaign to end Greek independence forever.
Remembering well the lessons at Marathon, Xerxes took his time to build a sizable force. Though some historical accounts tell of a force up to 2.5 million strong- these are almost certainly gross exaggeration, and it's more likely that Xerxes marched with 200,000 to 250,000, though for the ancient world this would certainly have been an incredible and mind-boggling number. Xerxes plan was simple: march into Greece through the north, and outflank any Greek defenders by landing his navy behind them along the Greek coast.