To protect themselves from attackers.
The armies and navies of two dozen southern Greek city-states, led at first by Sparta and later by Athens.
It was a sea battle - the southern Greek city-state navies versus the Persian navies mainly from Egypt, the Asian Greek cities and Phoenicia.
Both had well thought out strategies and military tactics, and large effective armies and fleets. Both relied on amalgamating separate forces. Both had large Greek components to their armies and navies, the Macedonians who were Greek, led a unified Greek army and the Persians paid Greek mercenaries.
Most of the city-states of Ancient Greece were relatively close to the water, especially those found on Crete, Cyprus, the Dodecanese Islands, or Cycladic Islands. Greek city-states favored marine travel which was more reliable and cost-effective than land travel. As a result, many city-states had strong navies as opposed to having strong armies. (Sparta is the one major exception to that rule.)
The Persian army, and the armies of the Greek city-states.
The Persian army, and the armies of the Greek city-states.
The combined armies of the Greek city-states.
The combined armies of about 20 Greek city-states led by Sparta.The combined armies of the southern Greek city-states, led by Sparta.
Greeks became agile mariners.Most of the city-states were relatively close to the water, especially those found on Crete, Cyprus, the Dodecanese Islands, or Cycladic Islands. As a result, Greek city-states favored marine travel which was more reliable and cost-effective than land travel. As a result, many city-states had strong navies as opposed to having strong armies. (Sparta is the one major exception to that rule.)
Generally they were in opposition and sometimes at war. At times some of the Greek city-states were allies of Persia. At other times Persia ruled large numbers of Greek cities and tried to keep them peaceful with mixed success.
A dozen Greek city-states and the Persian expeditionary force.
The Persian expeditionary force plus its Greek allies versus the alliance of southern Greek city-states.