Once upon a time there was this place called Sparta, where everyone who lived there was a total badass. They were so badass that whenever one of their citizens said some stupid bullshit like, "Hey man Democracy kicks ass... I like reading books, petting small puppies and not violently killing people with pointy sticks that are on fire", they just shipped his pansy ass off to Athens or Thessaly or Bill's Gay Land to philosophize about the nature of the soul, pick fresh daisies and have sex with men. Sparta was one of the hardest cities ever built; It's men were all trained from birth to be highly-disciplined explosive killing machines of death who didn't fuck around and it's women were taught to raise strong children and encourage their husbands to die honorably in battle. In fact, all citizens were required to be able to break at least three wooden boards with their foreheads, another four with a well-placed judo chop and punch bullets out of mid-air with their bare hands. Anyone who failed this test was sent into exile.
Well one day some fucking jackass named Emperor Xerxes I of Persia got all pissed off at how awesome Sparta was so he sent a bunch of soldiers to invade it and burn all of Greece to the ground so that he could make Persia the most badass civilization in the known world. Well obviously this didn't sit well with the Greeks. They hate it when fuckers burn down their cities.
Anyways, Xerxes' army of about 200,000 dudes created an impractically large pontoon bridge across the Hellespont and arrived in Greece in 480 BC with the intention of kicking everyones ass and possibly even taking names while doing so. However, in order to get through the countryside and sack Athens, they needed to pass through a hugeass mountain pass known as Thermopylae. The Spartans knew this, and they had every intention of using this to their advantage.
The Spartan King Leonidas I hand-selected a force of 300 of the most hardcore balls-out warriors in Sparta to defend the pass at Thermopylae. Knowing that it was pretty much a suicide mission, all the men he chose to accompany him had sons that were old enough to take over as head of their families. When Leonidas' wife was like, "what shall I do while you're gone?", Leonidas was just tipped his helmet and said, "Marry a good man, and have good children", before puffing on his cigar and riding out into the sunset Clint Eastwood-style.
The Spartans arrived at the pass along with a force of about 4,000 other Greek troops. The Spartans were like "get out of here, losers" and sent the other troops home to organize the defense of Athens while the Spartans stretched out, did some jumping jacks and then oiled themselves up like professional wrestlers. The other Greeks got the fuck out of there, except for a contingent of about 600 Thespian citizen-soldiers who may not have been hard as hell, but were totally brave as shit.
Xerxes showed up and was like, "what the fuck is this shit?". He opened a parlay with the Spartan King.
Xerxes: What the fuck is this shit?
Leonidas: Eat me.
Xerxes: There are 200,000 dudes here and you've only got 300 pussies guarding this pass. You're so fucked. Just lay down your weapons.
Leonidas: Why don't you come and get them, bitch? We'll see who's a pussy when I shove all 200,000 of those knuckleheads up your ass.
Xerxes: Fuck you.
Leonidas: Good one, fag. I've heard better insults from retards.
Xerxes: That's it.
Xerxes got totally pissed and assaulted the Greek defenses but the Persian light infantry was ill-equipped and way too lame to dislodge the battle-hardened Spartan warriors. They were torn to shreds by huge spears of the Greek phalanx and then put into a huge water balloon launcher and catapulted into the Aegean Sea. Xerxes finally ordered the light infantry to pull back and moved his archers to the front to fire a couple of volleys at the Greeks. One Spartan remarked that it was all good because there were so many arrows it was like they got to fight in the shade for a while. Seeing this was unsuccessful as well Xerxes brought in his bodyguard contingent of 10,000 Immortals, an elite unit in his army that was world-renowned for its ass kicking skills. The Spartans and Immortals clashed in fierce combat, and Xerxes could not believe it when he saw his most disciplined and experienced troops getting seriously fucked up by Leonidas' men.
At the end of the second day of battle, the Greeks still held the pass and the Persians had lost a shitload of soldiers. Unfortunately for King Leonidas, a Greek traitor defected to the Persians and told Emperor Xerxes about a secret way around the mountain pass. During the night, Xerxes sent what remained of his Immortals around to the other side of the pass and when the Spartans got ready to fight in the morning they noticed that there were now Persians on both sides of them.
Xerxes: Alright, bro. Nice try, but now you're toast.
Leonidas: Come on down here then. I've got a present for you.
Xerxes: Dude, you have no chance to survive. Just give up.
Leonidas: I've got a better idea. How about you shut the fuck up and bite my dick? That sound like a good plan?
So the Persian forces assaulted the Greeks from both sides. The Greeks held out for far longer than they should have, throwing themselves against the onslaught and battling with whatever they had -- spears, swords, fists, teeth, sticks, knees to the crotch... you name it. Eventually the last of the 300 Spartans fell to the Persian army as they tried to defend the body of their fallen King. The Persians found the body of Leonidas I, beheaded it and crucified his body outside the pass.
It takes a special kind of person to volunteer for a suicide mission and battle to the end against impossible odds. Leonidas found three hundred men willing to do just that. They did not fail the honor of their city or their people, willingly and unflinchingly facing an entire army to defend their homeland. The 300 Spartans (and 600 Thespians) held the mountain pass against impossible odds, being slain to the man but inflicting an estimated 30,000 casualties on the Persian army's most capable units. They dealt a serious blow to Xerxes' forces and managed to hold back the Persian advance long enough for the Greeks in Athens to prepare for a major military engagement at Salamis, where they would crush the Persian navy and effectively end the invasion of Greece.
If you're interested in reading a no-bullshit account of one of the greatest last stands in history, check this.