So guess what, it turns out that 300 was actually based on a true story…
…but you know that, of course. The Battle of Thermopylae was fought between the invasion forces of Persia under Xerxes I, which numbered about 200,000 or so, and an alliance of Greek forces under Leonidas I of Sparta. The Greeks held a narrow pass – the Thermopylae, or “Hot Gates” – that formed a natural choke point. (Indeed, as recently as 1941, it was used in a similar way by Greek and British Commonwealth forces to slow the Nazi advance.)
On the third day of the battle, the Greek forces realised that they were on the verge of being out-flanked by the Persians. The Phocian contingent, who guarded the passes, withdrew. Leonidas ordered his 300 members of the Spartan Royal Guard to stand and fight, and advised the rest of his allies to withdraw also.
700 Thespians, 400 Thebans, and assorted others, including Spartan helots, also stayed. Although the Greeks lost the battle, the larger strategic victory was theirs: they had slowed the Persian advance into Greece, allowing time for other forces to gather, and although 2000 men were lost on the Greek side, they inflicted casualties ten times that number upon the invaders.