Undoubtedly, one of the most powerful emotions, if not the most powerful, is fear. Especially for a ruler, fear can become the greatest weapon. But a weapon's barrel can also turn against its wielder.
Neonidas knew this very well. That's why, on his way to the capital, he had completely wiped out every town and village that resisted him. However, those who submitted to him were left untouched. In fact, he even engaged in trade with them.
The message was clear: the imperial army would hang those who opposed it but take good care of those who submitted. As a result, by the time Neonidas' army reached the capital, almost all cities, fortresses, and even remote divisions of the imperial army had swiftly switched to his side.
Of course, there were still those loyal to their king, or those who believed that God would miraculously save them. But to Neonidas, such beliefs were nothing more than laughable foolishness.