Phalanx, in ancient warfare, a close, deep formation of heavily armed infantry. It was the basic unit of ancient Greek armies. The phalanx soldier, called a hop-lite, carried a large shield, a short sword, and a pike (a long thrusting spear). In most battles before 400 B.C. two opposing phalanxes simply hurled themselves against each other.

Philip of Macedon (382–336 B.C.) revolutionized phalanx tactics by having the pikes thrust forward and held motionless so as to form an impenetrable hedge. By marching forward in close order, the phalanx forced the enemy to give way. Alexander the Great applied his father's tactics brilliantly in his conquests. In lesser hands the phalanx was vulnerable because of its lack of maneuverability. The phalanx became obsolete after the Battle of Pydna (168 B.C.), where the Romans destroyed the Macedonian forces demonstrating the superiority of the legion, with its more flexible organization, over the phalanx.