Why did the Greeks manage to defeat the huge Persian power?

The Greeks during the time of the Greco-Persian wars fought against the conqueror, they fought for their native land. In addition, during the war, the Greeks were able to rally and with common forces to repulse the formidable enemy. So, for example, in 79 BC there was a major battle near Plateia. There the Greeks, despite the numerical superiority of the enemy, were able to inflict a crushing defeat on the Persians. Thus, Xerxes’ campaign in Greece suffered its final collapse.