Well, if it's a Sparta analouge:
Only 1 out of 10 citizens really need to be in the military, or citizens at all. The rest could be slaves. Alright, they probably wouldn't be slaves in the modern since of the word, so...
Have it be a monarchy, or oligarchy, where about 1/10 people can trace noble heritage, and all those with royal heritage have to serve in the military.
The rest of the people don't have much say in the affairs of the state, but are diversified. And it'd have to be more than a simple city state, but Sparta could be it's capitol city and hold about 1/2 to a 1/3rd of the population...
It'd work out better for them if they found oodles of oil under sparta, or something, so the ruling elite don't have to depend on a tax base for their money, but instead just pump it out of the ground. That way they can ignore and oppress their people better.
With oil money, and good relations with other states, they could afford to buy top notch military equipment and punch above their weight....but if they're like the spartans, they'd be more concerned with keeping their people down than fighting other nations. But maybe they've whalloped the Turks once or twice, or something, so have earned a rep...
All supported by the finest oil and diamond and gold mines...which the ancient greeks never discovered...but modern tech and a druken sheppard did around 1920