The Battle of Ipsus could widely be considered as the definitive battle of the Wars of the Diodachi. The result was the defeat and death of Antigonus, who was incredibly successful up until that point, and his kingdom centered in Asia Minor.
The battle took place near the city of Ipsus on a wide plain in 301 BC. Although this is nearly three decades before EB's start date (and indeed I think it is about the limit that one could go outside of EB's time frame), it seems that many of the innovations that lead to the versions of the Hellenistic units that we know and love began a little before this time. The best example I can think of is the increased size of the sarissa used by the phalangites. In fact, the only major anachronistic element I can think of is the Celtic longsword used by Hypaspists, which was incorporated into their equipment after contact with the Galatians. If you are willing to look past this you hopefully should enjoy this battle.
The only full record of the battle comes from Plutarch's Life of Demetrius (relevent chapters are 28-30). Sparse fragments of the battle as described by Diodorus Siculus survive and indeed provide important details, but the numbers come Plutarch.
As you were reading that you probably noticed that the numbers are general, to the say the least. So how should army composition be decided? Well, I think I have come up with a method that is at least acceptable. This method is to use numbers from the battles of Alexander to derive percentage-compositions of armies, which can then be used to approximate the units used by later successors. What also seems to be quite important is that this battle takes place at a time before the economic hardship of constant fighting brings (even if it was lax). Antigonus still had massive amounts of funds available from his raiding of the treasury at Susa and the army under Seleucus was a coalition of several other successor powers. Ergo, elite units like the Pheraspidai and Hypaspistai will be amongst those at the battle.Antigonus
70,000 infantry
10.000 cavalry
75 elephants
Seleucus
64,000 infantry – at least 20,000 of which were light troops brought by Seleucus
10,500 cavalry (Possibly 15,000, but this latter value could be a misprint by Bar-Kochva)
400 elephants
120 chariots, which may or may not have been actually used in the battle
For the Antigonid forces I am using the numbers from Gaugamela as it took place relatively soon into Alexander's campaigns and with minimal influence from the interior of the Persian empire. Simply, this is a matter of the Antigonid kingdom's proximity to Greece. Conversely, the Seleucid forces have the Battle at the Hydaspes river in India as their model.
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