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. Men in rows behind the initial five angled their spears at a 45 degree angle in an attempt to ward off arrows or other projectiles. 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 arrive for several more days. Phalangites were drilled to perform short forced marches if required.
The Macedonian phalanx also proved to be one of the best defensive formations in all of antiquity thanks to its elongated spear (from 5 meters long during Alexander's reign to 7.5 meters during the 3rd and 2nd century BC) called a sarissa, and its very tight formation. The sarissa allowed the phalangites to keep the enemy out of range, tirelessly and effortlessly pushing back the opposing forces' charges and breaking every frontal assault of both cavalry and infantry with unmatched effectiveness while taking nearly no casualties. The phalangites could stick the bronze tip of their sarissa to block the most powerful enemy charges (mostly cavalry's, but sometimes chariots' and elephants') and they had the reputation to be almost invincible on frontal assaults. They were the ideal troops to hold a position as they were able to push their opponents back and keep them out of range for a nigh indefinite amount of time as long as they kept good cohesion. During the siege of Atrax (Thessaly) by the Roman legions in 198 BC, the Macedonian phalanx proved to be nigh impenetrable when charged up front, even by the best trained soldiers of Rome. The Romans managed to break into the city after breaching its walls, but were then faced with a compact formation of Macedonian levy phalangites. The Romans tried to get in between the pikes but were eventually pinned down by the sarissas of the soldiers right behind the front row, and never managed to reach their opposition with their short gladius.
The Macedonian phalanx dominated the European, Northern African and Asian battlefields from southern Italy and Sicily to modern Turkey, Egypt, Syria and Persia all the way to the western Indian frontiers for about two centuries, remaining totally unbeaten by any non-Macedonian army from its creation in 358 BC to the battle of Cynoscephalae in 197 BC (Beneventum is more of a tactical retreat).
The phalanx formation also had its disadvantages. Though practically unbeatable when charged from the front, Macedonian phalanx was very weak on its rear and flanks, mainly because of their very light armour and shield which offered very low protection while fighting in mêlée combat. Lighter troops like peltasts for example protected the flanks of the phalanx to prevent any dangerous enemy encircling maneuver to succeed.
The wars of the Diadochi, following the death of Alexander the Great and his empire's collapse, resulted in the first battles between identically trained and armed phalanges. The engagements between two Macedonian phalanges depended only on the number and the quality of the troops engaged and proved to be very long, indecisive and deadly. The sarissae tended to be prolonged in order to gain an advantage over the opposing phalanx, thereby inaugurating an arms race that could only be limited by the phalangites' stamina and the resistance of the weapon itself. The length of the pike was increased by half during the fifty years that followed the death of Alexander, reaching 7.5 meters during the siege of Edesse by the Spartans in 274 BC. The armour and shield of the foot soldiers got heavier through time as well, for their enemies weren't hoplites or Persian light infantry any more, but other Macedonian phalanges that could reach them with their equally long pikes. This new equipment was very uncomfortable for combat. But because the need of mobility and flexibility wasn't a concern any more at that time as the whole known world used increasingly heavy Macedonian style infantry, the phalanges renounced their mobility and their effectiveness against more flexible troops for better efficiency during phalanx engagements.
The longer sarissa became a problem, the mobility of the soldiers and of the sarissa was now extremely limited due to the excessive length of the weapon. The later phalanx had to fight in perfectly ideal conditions because it wasn't capable of reacting as fast as it used to. Unlike the earlier Macedonian phalanx, which fought in the harsh climatic and geographic conditions of the East, vastly outnumbered but always victorious, the later phalanx had now a very fragile cohesion. The heavier phalanges faced issues that Alexander's phalanges never experienced, which can only be explained by the fact that their equipment was now too heavy to fight in good order. According to Plutarch, sarissas of 5 meters seemed light and handy after a very long training, but a 7.5 meters sarissa, no matter how intensively prepared were the soldiers, could never be wielded with ease. Phalangites couldn't maneuvre freely as they used to, their pace was greatly reduced and they had to fight on perfectly flat battlefields in order to maintain a tidy formation. During the battle of Pydna, the phalanx formation collapsed because of the uneven terrain on which they were fighting. After pushing back and steam-rolling through the Roman legions, the phalanx had to pursue the retreating Roman infantry on the muddy hillsides behind the Roman army. The phalangites, cluttered by their excessively long spears, slipped and disorganised, ending up by being massacred by their more flexible opponents. Such problems do not appear to have ever happened to the phalanx of Alexander that could cross a river (Battle of the Granicus, Battle of the Hydaspes river) during battle in good order, adopting risky formations in echelons for example (Battle of Gaugamela) while later phalanx proved to be, according to Livy, hardly capable to turn around.