I'm probably going to see the film 300 tonight. The movie deals with the battle of Thermopylae (~480 BCE). The number three hundred refers to the three hundred Spartans (along with seven hundred Thespians) who held a vital pass during the war between the Greeks and the Persians. By holding the pass and causing the Persians significant losses the Spartans and Thespians secured the retreat of their Greek allies and thus their ability to regroup and fight another day. Significant Greek victories against the Persians followed this battle (indeed they pushed Persia out of Greece and thus ended the war), therefore sealing the importance of Thermopylae and the sacrifices of the men there.
Thermopylae also of course helped secure Europe (for the time being) from an Asian conquerer, and a number of scholars see this as an important factor in the continuing development of European culture.
Addendum: Upon reading a few more reviews I've decided not to see this film.