In 320, Seleucus I Nicator had been appointed as satrap of Babylonia, but Antigonus Monophthalmus had expelled him in the summer of 316. In May 311, however, Seleucus recaptured Babylon.
This was the beginning of the Babylonian War, which was to last until the late summer of 309. Antigonus first sent his son Demetrius against one of the largest cities of the world. He had to retreat, and because later Antigonid were equally unsuccessful, Seleucus could keep his newly-founded independent kingdom.
The story is told by Diodorus of Sicily (World History, 19.90-93); the translation was made by Russel M. Geer.
The beginning of the Babylonian war
[19.90.1] In Asia, after the defeat of Demetrius at Gaza in Syria,note[In the Autumn of 312, Ptolemy of Egypt had defeated Demetrius near Gaza. After this victory, he had sent Seleucus to the east.] Seleucus, receiving from Ptolemy no more than 800 foot soldiers and about 200 horse, set out for Babylon. He was so puffed up with great expectations that, even if he had had no army whatever, he would have made the expedition into the interior with his friends and his own slaves; for he assumed that the Babylonians, on account of the goodwill that had previously existed,note[Seleucus had been satrap of Babylonia between 320 and 315. He had behaved himself as a local king, which must have meant that he took part in the Akitu festival, which must have endeared him to the Babylonians.] would promptly join him, and that Antigonus, by withdrawing to a great distance with his army,note[He wanted to invade Europe.] had given him a suitable opportunity for his own enterprises.
[19.90.2] While such was his own enthusiasm, those of his friends who accompanied him were no little disheartened when they saw that the men who were making the campaign with them were very few and that the enemy against whom they were going possessed large armies ready for service, magnificent resources, and a host of allies.
[19.90.3] When Seleucus saw that they were terror-stricken, he encouraged them, saying that men who had campaigned with Alexander and had been advanced by him because of their prowess, ought not to rely solely on armed force and wealth when confronting difficult situations, but upon experience and skill, the means whereby Alexander himself had accomplished his great and universally admired deeds. He added that they ought also to believe the oracles of the gods which had foretold that the end of his campaign would be worthy of his purpose;
[19.90.4] for, when he had consulted the oracle in Branchidae,note[Seleucus had visited Miletus and the oracle of the Branchidae at the beginning of the Third Diadoch War.], the god had greeted him as King Seleucus, and Alexander standing beside him in a dream had given him a clear sign of the future leadership that was destined to fall to him in the course of time.
[19.90.5] Moreover, he pointed out that everything that is good and admired among men is gained through toil and danger. But he also sought the favor of his fellow soldiers and put himself on an equality with them all in such a way that each man respected him and willingly accepted the risk of the daring venture.
[19.91.1] When in his advance he entered Mesopotamia, he persuaded some, of the Macedonians who were settled at Carae to join his forces, and compelled the rest. When he pushed into Babylonia, most of the inhabitants came to meet him, and, declaring themselves on his side, promised to aid him as he saw fit;
[19.91.2] for, when he had been for four years satrap of that country, he had shown himself generous to all, winning the goodwill of the common people and long in advance securing men who would assist him if an opportunity should ever be given him to make a bid for supreme power.
[19.91.3] He was joined also by Polyarchus, who had been placed in command of a certain district, with more than a 1,000 soldiers. When those who remained loyal to Antigonus saw that the impulse of the people could not be checked, they took refuge together in the citadel, of which Diphilus had been appointed commander.
[19.91.4] But Seleucus, by laying siege to the citadel amid taking it by storm, recovered the persons of all those of his friends and slaves who had been placed there under guard by the order of Antigonus after Seleucus' own departure from Babylon into Egypt.
[19.91.5] When he had finished this, he enlisted soldiers, and, having bought up horses, he distributed them to those who were able to handle them. Associating with all on friendly terms and raising high hopes in all, he kept his fellow adventurers ready and eager under every condition. In this way, then, Seleucus regained Babylonia.note[Between 13 May and 1 June 311.]
[19.92.1] But when Nicanor, the general in Media, gathered against him from Media and Persia and the neighboring lands more than 10,000 foot soldiers and about 7,000 horse, Seleucus set out at full speed to oppose the enemy.
[19.92.2] He himself had in all more than 3,000 foot and 400 horse. He crossed the Tigris; and, on hearing that the enemy were a few days' march distant, he hid his soldiers in the adjacent marshes, intending to make his attack a surprise.
[19.92.3] When Nicanor arrived at the Tigris and did not find the enemy, he camped at one of the royal stations,note[A station on the Royal road.] believing that they had fled to a greater distance than was the case. When night was come and the army of Nicanor was keeping a perfunctory and negligent guard, Seleucus fell on them suddenly, causing great confusion and panic;
[19.92.4] for it, happened that when the Persians had joined battle, their satrap Euagorasnote[The satrap of Aria.] together with some of the other leaders. When this occurred, most of the soldiers went over to Seleucus, in part because they were frightened at the danger but in part because they were offended by the conduct of Antigonus.
[19.92.5] Nicanor, who was left with only a few men and feared lest he be delivered over to the enemy, took flight with his friends through the desert. But Seleucus, now that he had gained control of a large army and was comporting himself in a way gracious to all, easily won over Elam, Media, and some of the adjacent, lands; and he wrote to Ptolemy and his other friends about his achievements, already possessing a king's stature and a reputation worthy of royal power.