The Battle of Ipsus was fought between some of the Diadochi (the successors of Alexander the Great) in 301 BC near the village of that name in Phrygia. Antigonus I Monophthalmus and his son Demetrius I of Macedon were pitted against the coalition of three other companions of Alexander: Cassander, ruler of Macedon; Lysimachus, ruler of Thrace; and Seleucus I Nicator, ruler of Babylonia and Persia.
Antigonus was 80 years old and the ruler of modern day Syria, Turkey, Lebanon, and Judea. He and his son Demetrius had generally had the better of the fighting in the wars running up to this point: The Siege of Rhodes, although an operational failure, was a victory for the Antigonids in that the Rhodians agreed to help them against everyone and anyone except for Ptolemy; Cassander had been largely neutralized by Demetrius and the Hellenic League; and Ptolemy was still recovering from the Antigonid invasion of 306. Their overall strategy in this fourth War of the Diadochi was to engage the various successors and defeat them in detail, and had so far been successful. Cassander, their only enemy still effectively resisting in 302, was nearly isolated, and his allies had not yet made a move to support him. Seleucus, especially, would have been a major help to Cassander, as he had recently exchanged some of his most eastern lands for 500 elephants from Chandragupta Maurya. However, seeing the plight of his ally, Lysimachus undertook to invade Asia Minor to distract the Antigonid armies fighting against Cassander, who was soon relieved of Demetrius' pressure as the latter moved his army to Anatolia to fight Lysimachus. Cassander himself soon was able to give assistance, keeping only seventeen thousand men with him to fight Demetrius in Thessaly, and together the two allies overran most of western Asia Minor.
Lysimachus, with the contingent from Cassander, was isolated in western Anatolia, on the other side of the Aegean from his base of supply in Europe. In stark opposition, Antigonus and Demetrius were now in their own territory, and their supply lines were far shorter. They also had 75 war elephants with which to support their cavalry and wreak havoc upon the allied phalanx. Confronted with far superior numbers, the allies fell back without major engagement. However, Cassander had previously planned a move by Seleucus to bring his vast numbers of elephants into the fray, and now his ally came from the east to engage Antigonus from the rear. Antigonus was unable to bring Lysimachus and Cassander to battle before Seleucus and his son Antiochus joined up with the allied forces. The united allied army, believed to be about 60,000 in number, faced Antigonus and Demetrius in Phrygia on an open plain well-suited for both the allied preponderance of elephants and the Antigonid superiority in cavalry numbers and training.
Except Plutarch's life of Demetrius, almost no histories have survived with an account of the battle.
Both sides deployed their phalanx in the center in formation echeloned to the left rear, as was normal among Alexandrine and Diadochi armies. On the allied side, Lysimachus and Cassander split their cavalry evenly between the two flanks, with 100 of Seleucus' elephants deployed in line, with the rest in reserve under his personal command. Lysimachus commanded the right flank cavalry and Antiochus was in command of the left. Light-armed troops, mainly peltasts and a few psiloi, were deployed to the army's front. On the other side, Antigonus placed his most and best cavalry, under Demetrius, on his right flank; he had greater numbers of heavy infantry, but apparently chose not to lengthen his line but rather to deepen the phalanx. He, too, deployed light-armed troops forward of his army.
The battle opened with the usual slowly intensifying skirmishing between the two armies' light troops, with elephants eventually thrown into the fray by both sides. Efforts were made by both sides to hamstring the enemy's elephants, but also had to hang back to protect their own. Demetrius' superior right-flank cavalry drove Antiochus' wing back, but was halted in his attempted rear blow by Seleucus, who moved the elephant reserve to block him. Lysimachus on the allied right made slow progress against the Antigonid troops on his wing, but had the foresight to detach some horse archers and skirmishers to the center, to carry the fight against the enemy skirmishers there. More missile troops moved to the unprotected Antigonid right flank, as Demetrius was unable to disengage from the elephants and enemy horse to his front. With control of the center of the field, the allied missile troops rained javelins and arrows down on the numerically superior Antigonid infantry, whose morale began to waver. Eventually they began to break, and streamed towards the rear, fleeing the enemy missile troops. Antigonus attempted to rally his troops and present more of a front to the enemy missile units and main phalanx. At the beginning of the day he had not been able to wear plate armor; this disadvantage was unexpectedly used by an anonymous allied peltast, who killed him with a well-thrown javelin. Without leadership and already beginning to flee, the Antigonid army completely disintegrated, with a fragment of the army surviving under Demetrius, who managed to escape the allied cavalry.
The last chance to reunite the Alexandrine Empire had now passed. Antigonus had been the only general able to consistently defeat the other Successors; without him, the last bonds the Empire had had began to dissolve. The world was carved up between the victors, with Ptolemy retaining Egypt, Seleucus receiving the bulk of Antigonus' lands in the east and eastern Asia Minor, and Lysimachus receiving the remainder of Asia Minor. Eventually Seleucus would defeat Cassander and Lysimachus (in 281 BC), but died shortly afterward. Ipsus finalized the breakup of an empire, which may account for its obscurity; despite that, it was still a critical battle in classical history and decided the character of the Hellenistic age.
Battle of Ipsus (http://www.ancientlibrary.com/wcd/Battle_of_Ipsus)
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