Which things united the Greek city-states

One powerful reason for unity among Greek city-states was external threats. The threat of invasion and domination by Persia was a strong motivation for different Greek states to join forces and fight together large invading forces of the mighty Persian Empire. Those conflicts are called the Medic Wars or the Greco-Persian Wars. The most intense fighting took place during the two invasions of mainland Greece launched by Persian rulers between 490-479 BCE.