Yes I know you watched 300. A tiny number of Greeks really whipped all of Persia.
The Persians were weaker & dumber. But the Romans were not.
The Romans did beat & suck in the Spartans. They became Romans. So we know who won.
The Spartans were tough; strong; athletic; fit, fanatics.
But they never adapted. Their foot techniques were great in certain settings. The Persians were a rabble slave army led by morons. They let the Spartans choose how to fight. Rule one of war; you choose how the battle is fought.
The Spartans had poor armor; no archery; no heavy horse units; no quick mounted units for recon & quick attacks at rear.
The Romans were far superior to the Spartans.