One side in the Battle of Thermopylae (8)

I believe the answer is:

spartans

(Other definitions for spartans that I've seen before include "Harshly treated citizens once" , "These old Greeks lived frugally and harshly" , "hardy fellows" , "Hardy old Greeks" , "Hardy and frugal citizens of old Greece" .)

I've seen this clue in The New York Times.

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