Plenty of places in ancient times used terror successfully to keep large slave populations in their place. But if you count up Sparta's win/loss record in known battles against other armies, they're almost exactly 50%. And they were 0-2 against the Macedonian phalanx.
Kind of the opposite to Napoleon. Sparta fought one famous battle and is seen as invincible, Napoleon won almost all his battles, lost one decisive battle at Waterloo (incredibly close) and since then is seen as a looser. Propaganda is powerful tool.