Why did Greece, a small country, moreover divided into dozens of states, manage to resist the powerful Persian power?

This is due to the fact that the Greeks united at the last moment, and therefore, they were able to repulse such a formidable and powerful enemy as the Persian Empire. The very first major victory for the Greeks was in 490 BC in the Battle of Marathon. Here the 25 thousandth army of the Persians was thrown back into the sea. After that, other policies joined Athens.