300/WMG

This is Sparta.
Why would Leonidas lie right before his Crowning Moment of Awesome? Also, thousands of Poopers can't be wrong.

Xerxes is a Goa'uld
He wears a lot make up, he wears a lot of jewelry and thinks he's a god. Oh! and he also has a very deep raspy voice.
 * All we need now is for his eyes to start glowing and it's confirmed.

300 takes place in the realm of Greek Myth.
This explains all seeming historical inaccuracies: This is really what the Battle of Thermopylae was like in a shadowy otherworld where the Spartans really were descended from Heracles and the Persian Army really did contain rocket-wizards and giant rhinos and immortal ninjas and was lead by an actual god.
 * This is sort of explicitly shown at the end of the movie, where the One-Eyed Spartan is telling the story of Leonidas to an army of Greeks.
 * It could also simply be the One-Eyed Spartan's version of events. He's the only one alive to tell about it (well, the only one on the "good" side), so perhaps it's just a fisherman's tale.
 * Dilios being an Unreliable Narrator is canon, so things didn't really happen like that in-verse.

It was all just an incredibly lavish LARP campaign...
... seen through the eyes of the geeks participating. Hot babes, lots of sex, lots of homoerotic fightin' men, and immense battles from which much EXP must have been reaped. Clearly this was all just a big LARP session, completely with cinematic slow-mo from the guys who always go "I decapitate that one dude in Bullet Time."
 * Given that the movie adds so many things that weren't in the book, this actually kinda makes sense. The only snag is the council subplot; roleplaying tedious political drama generally isn't too fun.
 * Obviously, sir, you have never larped Requiem.

300 is real, what History tells you is wrong.
Because real life can be awesome like that. Also, it would explain where ninjas came from. Or not...

300 is the past of Naruto.
Well, it's ridiculous, but yet, bullet time and hyperspeed seem to be constants in both worlds. Not to mention flamboyant/overly pierced ninjaesque villains.

300 takes place inside The Matrix
C'mon, who would believe that an army of Badasses killed over eight thousand Persians in bullet time is historical?
 * That means Leonidas is The One. Whoa!

The vegetation in Persia is the natural ancestor of steroids.
Which would explain where the Persian army got those seemingly absurd elephants and rhinos. If this theory is true, it could also be that the mutant wolf killed in the beginning was a tourist from Persia where steroid-infected herbivores are the local carnivore cuisine. Oh, and the slaughter of an innocent Persian tourist - it just wanted to say an affectionate hello, truly! - could be the real reason for the war between the Spartans and the Persians.
 * Anabolic steroids are simply synthesized human testosterone. Less steroids, more 50s style Science! radiation plaguing the plants or something.

Spartacus takes place in the same world.
Everyone I know has been complaining that Spartacus: Blood and Sand is 300 and Gladiator tossed in a blender. I contend that Spartacus actually takes place in the same universe as 300. Spartacus is perhaps, a distant descendant of a Spartan warrior who fought in the Persian Wars.

300 takes place in the Warhammer 40,000 setting
This might be a bit of a stretch but I've been thinking about it. The world where 300 takes place is a Space Marine recruitment world, probably designated a fuedal world rather than feral or death. The reason that no Astartes appear? It isn't their homeworld, so they don't have a permanent presence, just return every generation or so to oversee aspirant trials. They've set up the myths and legends to create a world with greek-inspired warriors, complete with entire other nations for them to pit themselves against. Unfortunately, during the absence of the SM the Persian king and followers have become corrupted by Chaos, probably Slaanesh as excess seems to play quite a part in their appearance in the film. After all, excess is more than just about sex, and Xerxes wants to have the biggest empire, the biggest army, he simply wants it all! It also explains the presence of purple goat/beastmen, strange rhinos, etc. And then there is the big fat executioner with blades for arms, that has to be either a chaos ogryn or a mutant of some sort.
 * If he is Slaanesh, then why is Xerxes a God-Emperor?
 * Simple, he's got delusions of grandeur, like a lot of human cultists. He's not a traitor marine, he's from that planet, it's quite likely his knowledge of the wider galaxy is next to nothing since it's a feudal world. He doesn't even realise he's been corrupted by chaos, despite appearances he is just a pawn like every other mortal who falls under the sway of chaos.
 * ...You just gave me inspiration to design a new chapter, cheers mate.
 * Glad I could help, craziness pays off occassionally.
 * The Spartans are ridiculously bloodthirsty and wear red cloaks and brass/bronze helmets. Perhaps they have been unwittingly corrupted by MIGHTY KHORNE?

Tengen Toppa Gurren Lagann takes place in the same world.
And this is even more screwed up than I thought. After all, how do you explain away all the Testosterone Poisoning?
 * You mean this?
 * Spartan = Spiral?
 * That's why Xerxes (an agent of the Anti-Spirals) wanted Sparta so badly, the Spartans are going to cause.

Stelios is Azazeal from Hex.
They share the same actor, and we know that Azazeal has been trying to father the antichrist since at least Ancient Egyptian times.
 * Corollary: Queen Gorgo is a witch.
 * Nah, he'll have been after the Ephors' oracle!

Ephialtes' father was a male, soldier equivalent of the Education Mama.
Ephialtes told Leonidas that it was his mother's love that saved him from being killed as a baby, not his parents' love. The mother probably pressured her husband into escaping with them. Humiliated by the loss of his career and his deformed child, the father began obsessively training his son as a soldier. This causes Ephialtes' obsession with becoming a soldier.

The Persian messenger who gets kicked into the Spartan well by Leonidas is originally from Persian-occupied Egypt, and of Nubian descent.
"Okay", you may now say, "this would explain this one black Persian. But why is every other high-ranking Persian - including Xerxes himself - also black?" Because the Messenger is the only Persian whom all the Spartans who where not among the 300 have ever seen up close. This means that when the last survivor of the 300 tells his story, this is what his audience imagines all the Persians to look like.

The story takes place in an Alternate History where Persia has conquered most of the Eurasian landmass
Hence why they had ninjas.