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The Macedonian phalanx is an infantry formation developed by Philip II and used by his son Alexander the Great to conquer the Persian Empire and other armies. Phalanxes remained dominant on battlefields throughout the Hellenistic period, although wars had developed into more protracted operations generally involving sieges and naval combat as much as field battles, until they were finally displaced by the Roman legions.
Philip II spent much of his youth as a hostage at Thebes, where he studied under the renowned general Epaminondas, whose reforms were the basis for the phalanx. Phalangites were professional soldiers, and were among the first troops ever to be drilled, thereby allowing them to execute complex maneuvers well beyond the reach of most other armies. They fought packed in a close rectangular formation, typically eight men deep, with a leader at the head of each column and a secondary leader in the middle, so that the back rows could move off to the sides if more frontage was needed.
Each phalangite carried as his primary weapon a sarissa, a double-pointed pike over 6 m (18 ft) in length. Before a battle the sarissa were carried in two pieces and then slid together when they were being used. At close range such large weapons were of little use, but an intact phalanx could easily keep its enemies at a distance; the weapons of the first five rows of men all projected beyond the front of the formation, so that there were more spearpoints than available targets at any given time. The secondary weapon was a shortsword called a kopis, which had a heavy curved section at the end.
Neither Philip nor Alexander actually used the phalanx as their arm of choice, but instead used it to hold the enemy in place while their heavy cavalry broke through their ranks. The Macedonian cavalry fought in wedge formation and was stationed on the far right; after these broke through the enemy lines they were followed by the hypaspists, elite infantrymen who served as the king's bodyguard, and then the phalanx proper. The left flank was generally covered by allied cavalry supplied by the Thessalians, which fought in rhomboid formation and served mainly in a defensive role.
Other forces — skirmishers, range troops, reserves of allied hoplites, archers, and artillery — were also employed. The phalanx carried with it a fairly minimal baggage train, with only one servant for every few men. This gave it a marching speed that contemporary armies could not hope to match — on occasion forces surrendered to Alexander simply because they were not expecting him to show up for several more days. Phalangites were drilled to perform short forced marches if required.
The armies of the early Hellenistic period were equipped and fought mainly in the same style as Alexander's. Towards the end, however, there was a general slide away from the combined arms approach back to using the phalanx itself as the arm of decision, having it charge into the enemy lines much like earlier hoplites had. This left the formation fairly vulnerable — though near invincible to frontal assault, phalanxes were susceptible to flanking, and tended to break formation when advancing quickly over rough ground. So long as everyone was using the same tactics these weaknesses were not immediately apparent, but with the advent of the Roman legion they proved fatal in every major engagement, the most famous being the Battle of Pydna, as the Romans were able to advance through gaps in the line and easily defeat the phalangites in close quarters.
The Macedonian phalanx was not very different from the Hoplite phalanx of other Greeks states, it was better trained, armed with the sarissa enabling it to outreach its competitors and stave off enemy cavalry, and wore far lighter armor enabling longer endurance and long fast forced marches, including the ability to sprint to close and overwhelm opposing positions and archers. In essence, the range of their counter-weighted sarissa, allowed them superior mobility as well as superior defense and attack abilities despite the encumbrance disadvantages of the longer weapon once trained up to handling it in formation. Centuries later, the organized militia of Swiss pikemen enjoyed similar advantages over less well trained contemporary militaries which were identically equipped, which emphasizes the importance of training and unit cohesion in the scheme. In Phillip's and Alexander's time, the Macedonian phalanx had clear technical superiority.
Attempts for revival in the Roman period 
Some Roman emperors were great admirers of Alexander of Macedon and attempted to imitate the Macedonian Phalanx. This is attested by Cassius Dio, Herodian and the Augustan History. Emperor Caracalla (2nd-3rd century AD) had formed a Macedonian phalanx of 16,000 men all born in Macedonia and commanded by officers bearing the names of Alexander's officers. Also Severus Alexander (3rd century AD) had an imitation of the Macedonian phalanx of 30,000 "phalangarii" and with this he won many victories in the Persian Wars.