Asthetairoi

The pezhetairoi (Greek and Ancient Macedonian: πεζέταιροι, singular: pezhetairos) were the backbone of the Macedonian army and Diadochi kingdoms. They were literally "foot companions" (in Greek, pezos means "foot warrior" or "infantryman", and hetairos means "companion" or "friend").

The Macedonian phalanxes were made up almost entirely of pezhetairoi. Pezhetairoi were very effective against both enemy cavalry and infantry, as their long pikes could be used to impale enemies charging on horse-back or to keep enemy infantry with shorter weapons at bay.

The pezhetairoi were the battalions of the Macedonian phalanx. They first came to prominence during the reign of Philip II, particularly when they played such an important role in Philip's subjugation of Greece at the Battle of Chaeronea in 338 BCE. The name "foot companions" was used by famed commander Alexander the Great, the successor of Phillip II, as an ancient recruiting method. By labeling his infantry as his personal "companions" and "friends," Alexander was able to engage a wider manpower base for his subsequent military campaigns, as positions in his personal infantry would denote pride and honor. Therefore, as Alexander would campaign across Asia, he was able to use his reputation as a brilliant military strategist, along with the personal connotation given to his infantry, to recruit more native peoples to his cause than conventional recruitment methods of the day. This created a near endless base of manpower for the burgeoning military leader, and allowed him to engage in longer, costlier campaigns in areas like Asia Minor, Egypt, Persia, and India.

They were armed with the sarissa, a long spear with a shaft made from flexible cornel wood, which had a much longer reach than the traditional hoplite spear. Because of its length the phalanx could present the spearpoints of around five files of men; which made the phalanx almost impenetrable, and fearsome to oppose.

Tactically, the pezhetairoi were best used as a strong defensive line, rather than as shock troops. The length of the sarissa, while making them terrifying for an enemy to oppose, severely limited their maneuverability; and if they were taken in flank or rear they had little chance of responding. This was particularly clear at the Battle of Gaugamela in 331 BCE, when the rapid advance of the right wing caused a breach to open between two of the battalions of pezhetairoi—a force of enemy cavalry broke through and, had it not been for a lack of discipline in their own command, and for Alexander's placing of a second line of traditional hoplites in reserve, the phalanx might have been destroyed from the rear.

...
Wikipedia