Ancient Greek Warfare
Greek soldiers (Chigi vase)
Wars were very common in ancient Greece. The Greeks lived in little city-states, each one like a small town in the United States today, with no more than about 100,000 people in each city-state. These city-states - Athens, Sparta, Corinth, Thebes - were always fighting each other over their borders. Often they would get together in leagues, a lot of city-states together, to fight as allies.
Sometimes other people invaded Greece, and then there would be wars to defend the city-states from the invaders. Sometimes the city-states fought together, and sometimes they didn't.
Then again, sometimes the Greeks fought in other countries. They invaded other countries and took them over, or they raided other cities and took their stuff. And they often fought for pay, as mercenaries, when one foreign country fought another (more on this here).
Although there were many wars in ancient Greece, most of them we don't know very much about. There are four main wars that we do know about, thanks to the writing of Homer and Herodotus and Thucydides and Arrian. These are the Trojan War (about 1250 BC, but it may be mainly a made-up story), the Persian Wars (490-480 BC) and the Peloponnesian War (441-404 BC) and the campaigns of Alexander the Great (331-323 BC). Then a little later, Greece was taken over by the Romans (146 BC), which we know about thanks to Polybius.
To find out more about Greek warfare, check out these books from Amazon.com or from your library:
Ancient Greek War & Weapons (People in the Past), by Haydn Middleton (2002). For kids.
Greek Hoplite (Soldier Through the Ages), by Martin Windrow (1985). For kids, from Scholastic.
Greek Hoplite 480-323 BC, by Nicholas Sekunda (2000). From Britain. A good first guide, useful for painting models or illustrating reports.
Greece and Rome at War, by Peter Connolly (1998). Not for kids, but kids do like it.