Seleucus I's intensive military activity after the occupation of Babylonia in 312 B.C., which laid the foundations of the Empire, is recorded only once, and this is in the account of his part in the battle of Ipsus (301 B.C.), which decided the dispute over Alexander's domain between Antigonus and his son Demetrius, who were in favour of unification, and the former generals Lysimachus, Cassander, Seleucus, and Ptolemy, who preferred to see the Empire divided among themselves. Although Seleucus' troops, who actually won the day, comprised only one part of the Allied army, which was mainly based on Lysimachus' infantry and a detachment sent by Cassander, the course of the battle, unique as it was, is reminiscent in its general outline of the tactics of the later Seleucids.

In fact only part of the battle can be reconstructed. The main source, Plutarch in his biography of Demetrius (28–30), focuses his interest, as may be expected, on Demetrius' activity and the tragic fate of Antigonus, paying scant attention to the military manoeuvres. His dramatic treatment of the event and its tragic features indicate that he relied here, as in other biographies, on Duris of Samos, whose emotional and sensational style must be approached with caution. But even if Plutarch had based his account of Ipsus on Hieronymus of Cardia, an eyewitness and experienced soldier whose report was certainly excellent, Plutarch's own lack of understanding of military affairs, his frequent distortion of reliable military authorities, and his tendency to impose his own conception of his hero's character on the facts, must be borne in mind when evaluating his interpretation. Diodorus' version, presumably paraphrasing Hieronymus of Cardia, was lost.