Antigonus sent his son Demetrius along with 15,000 infantry and 4,000 cavalry to reconquer Babylon.
Apparently, he gave Demetrius a time limit, after which he had to return to Syria.
Antigonus believed Seleucus was still ruling only Babylon.
Perhaps Nicanor had not told him that Seleucus now had at least 20,000 soldiers.
It seems that the scale of Nicanor's defeat was not clear to all parties.
Antigonus did not know Seleucus had conquered the majority of the eastern provinces and perhaps cared little about the eastern parts of the empire.

When Demetrius arrived in Babylon, Seleucus was somewhere in the east.
He had left Patrocles to defend the city.
Babylon was defended in an unusual way.
It had two strong fortresses, in which Seleucus had left his garrisons.
The inhabitants of the city were transferred out and settled in the neighbouring areas, some as far as Susa.
The surroundings of Babylon were excellent for defence, with cities, swamps, canals and rivers.
Demetrius' troops started to besiege the fortresses of Babylon and conquered one of them.
The second fortress proved more difficult for Demetrius.
He left his friend Archelaus to continue the siege, and himself returned west leaving 5,000 infantry and 1,000 cavalry in Babylon.
Ancient sources do not mention what happened to these troops.
Perhaps Seleucus had to reconquer Babylon from Archelaus.
Antigonus sent his son Demetrius along with 15,000 infantry and 4,000 cavalry to reconquer Babylon. Apparently, he gave Demetrius a time limit, after which he had to return to Syria. Antigonus believed Seleucus was still ruling only Babylon. Perhaps Nicanor had not told him that Seleucus now had at least 20,000 soldiers. It seems that the scale of Nicanor's defeat was not clear to all parties. Antigonus did not know Seleucus had conquered the majority of the eastern provinces and perhaps cared little about the eastern parts of the empire.
When Demetrius arrived in Babylon, Seleucus was somewhere in the east. He had left Patrocles to defend the city. Babylon was defended in an unusual way. It had two strong fortresses, in which Seleucus had left his garrisons. The inhabitants of the city were transferred out and settled in the neighbouring areas, some as far as Susa. The surroundings of Babylon were excellent for defence, with cities, swamps, canals and rivers. Demetrius' troops started to besiege the fortresses of Babylon and conquered one of them. The second fortress proved more difficult for Demetrius. He left his friend Archelaus to continue the siege, and himself returned west leaving 5,000 infantry and 1,000 cavalry in Babylon. Ancient sources do not mention what happened to these troops. Perhaps Seleucus had to reconquer Babylon from Archelaus.
Seleucus I Nicator