Nicanor was on route to recapture the city of Babylon from Seleucus, but he was defeated when Seleucus surprised him with an assault on his camp during the night, forcing Antigonus to cease hostilities with the other Diadochi, (Ptolemy, Cassander and Lysimachus) in order to concentrate his efforts on recapturing the city of Babylon himself.
Hearing of this news Nicanor, satrap of the east under Antigonus, quickly assembled an army composed of 10,000 infantry and 7,000 cavalry, with a large contingent under Eaugoras, satrap of Persia, and possibly the remaining Silver Shields who Antigonus had exiled to Arachosia in 316 BC.
After assembling his army he marched west, towards Seleucus who could gather no more than 3,400 soldiers, the majority of them infantry, so he decided to shadow Nicanor's movements and waited for him to cross the Tigris hiding in the marshy terrain around it.
[1] The news of Seleucus's victory came as a surprise in the west and prompted a shake-up of political alignments, ending the peace.
Diodorus of Sicily (World History, 19.90-93) https://www.livius.org/sources/content/diodorus/the-beginning-of-the-babylonian-war/ Antigonus the One-Eyed: Greatest of the Successors by Jeff Champions