Why did the citizens of the policies of Greece voluntarily go to the mortal battle?

In the 5th century BC, war broke out between Greece and the Persian Empire. The Persian kings sent their large armies to Greece. So one city after another fell under the onslaught of the conquerors. But some of the policies have united their efforts to resist the common enemy. In this connection, the Greeks soon managed to inflict a series of defeats on the Persians, where the battle of Plataea in 479 BC became key.