The Persians, as you say were advancing through Greece. The Athenians (playing the cut and runners in the movie) already pledged to stand against them. Theron was paid to convince the Spartan's to let the Persians pass. That makes him a war profiteer, kinda but not exactly like Haliburton. Eventually (way beyond the time frame of the movie) the Persians took Athens and the Athenian fled. The Athenians, however, tricked the Persian fleet into the straits of the Bay of Salamis (you can check the spelling, I'm just relying on my memory from classics class here) and won the decisive sea battle that Herodotus claims was the turning point of the war. Unlike the Persians, the boy loving, rational Athenians knew that it was better in both those circumstances (at Thermopylae and Athens) it's better for your country to live and fight when you can win.