What was a hoplite phalanx in ancient Greece?

A hoplite phalanx was the standard battle formation of heavily armed Greek citizen-soldiers called hoplites. It relied on close cooperation: each man protected part of his own body and the left side of the man beside him with a large round shield called an aspis. The formation was most effective on relatively open ground and became central to warfare in the Greek world from about the 7th century BCE onward. Famous battles such as Marathon in 490 BCE showed how decisive disciplined phalanxes could be. Later Macedonian armies adapted the idea with longer spears called sarissae.