One thing that never seems to get brought up in discussion of the battle of Thermopylae is that the Spartans also brought ~900 conscripted helots to fight for them (according to Herodotus, Diodorus Siculus estimated it closer to 1000). They were still totally outnumbered (combined forces of the city states was somewhere between 5,200-7,700 men compared to the 120,000 that fought for Persia).
But the bulk of Sparta's army being untrained slaves conscripted into fighting somewhat degrades the idea of this elite fighting force™ that works like 300 like to pretend they were.
There's so much emphasis on how the Spartans are the dangerous, tough, manly men, the only force that can save the West™ from the removed other.
My favourite scene for this is when Leonidas yells at the Greeks for not being professional soldiers, 'cos IRL some 400 of those would've been from Thebes, which not only also had a professional military, it had a larger and better trained one, and would already have defeated Sparta in battle multiple times by this point.