Why did Xerxes release the Greek infiltrators? Why did Greece manage to resist the Persian power?

In 480 BC, the Persian king Xerxes invaded the territory of Ancient Greece. The reason for the war was the intervention of the Greeks in the internal affairs of the Persians and revenge for the defeat ten years ago. Xerxes gathered a huge army for that time from all corners of his empire. To demoralize the Greeks, he released scouts, who, according to his plan, were supposed to tell about the huge army of the Persians and the futility of resistance. But it turned out exactly the opposite. For the Greeks, this war became national. Greek city-states defended their sovereignty and managed to win. The main factor here was the cohesion of all citizens of the country. In addition, the military organization of the Greeks turned out to be much stronger than the capabilities of the Persians. In addition, the Persian cavalry did not have the opportunity to deploy in mountainous Greece. These factors influenced the final victory of the Greeks in the war.