The Battle of Ipsus was a battle of the Fourth War Of The Diadochi between some of the Diadochi 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 Seleucus I Nicator, ruler of Babylonia and Persia, Cassander, ruler of Macedon; and Lysimachus, ruler of Thrace.

Seeing that Demetrius's war effort was aimed at destroying his power in Greece, and ultimately in Macedonia, Cassander tried to come to terms with Antigonus. However, Antigonus rejected these advances, intent on forcing Cassander's complete surrender. Cassander therefore held counsel with Lysimachus, and they agreed on a joint strategy that included sending envoys to Ptolemy and Seleucus, asking them to join in combatting the Antigonid threat. Seeking to take the initiative, Cassander sent a significant portion of the Macedonian army under Prepelaus to Lysimachus, which was to be used in joint operations in Asia Minor. Meanwhile, Cassander took the rest of the Macedonian army into Thessaly to confront Demetrius.

According to Plutarch, Antigonus's army before the battle numbered around 70,000 infantry, 10,000 cavalry and 75 war elephants, opposing 64,000 infantry for the allies, with 10,500 cavalry, 400 elephants and 120 scythed chariots.

Demetrius launched the principal Antigonid thrust, manoeuvering his cavalry round the elephants, and attacking the allied cavalry under Antiochus. Plutarch says that Demetrius "fought brilliantly and routed his enemy". However it is also clear that Demetrius allowed the pursuit of the routing allied cavalry to go too far, and resulting in his men becoming isolated from the battlefield. owever, Demetrius found himself unable to return to the battlefield because of the deployment of 300 elephants in his path. The ancient sources repeatedly emphasise the effect of elephants on horses, which are alarmed by the smell and noise of elephants and are loathe to approach them. Demetrius would not have been able to take his horses through the line of elephants, nor manoeuvre around such a large quantity of elephants. This elephant manoeuvre' was the decisive moment in the battle.

The morale of the troops appears to have collapsed, and it seems that some of the heavy infantry either defected to the allied side, or otherwise fled. Antigonus, stationed in the centre, tried to rally his men, hoping for Demetrius's return. However, he was gradually surrounded by allied infantry, and, he eventually killed by several javelins thrown by allied skirmishers. With the death of their commander, the Antigonid battle-line dissolved, and the battle effectively ended

The last chance to reunite the Alexandrine Empire had already been passed when Antigonus lost the Babylonian War and two thirds of his empire.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.