Alexander the Great died without a successor in Babylon on June 10, 323 BC.
His general Perdiccas became the regent of all of Alexander's empire, while Alexander's physically and mentally disabled half-brother Arrhidaeus was chosen as the next king under the name Philip III of Macedon.
Alexander's unborn child (Alexander IV) was also named his father's successor.
In the "Partition of Babylon" however, Perdiccas effectively divided the enormous Macedonian dominion among Alexander's generals.
Seleucus was chosen to command the Companion cavalry (hetairoi) and appointed first or court chiliarch, which made him the senior officer in the Royal Army after the regent and commander-in-chief Perdiccas.
Several other powerful men supported Perdiccas, including Ptolemy, Lysimachus, Peithon and Eumenes.
Perdiccas' power depended on his ability to hold Alexander's enormous empire together, and on whether he could force the satraps to obey him.

War soon broke out between Perdiccas and the other Diadochi.
To cement his position, Perdiccas tried to marry Alexander's sister Cleopatra.
The First War of the Diadochi began when Perdiccas sent Alexander's corpse to Macedonia for burial.
Ptolemy however captured the body and took it to Alexandria.
Perdiccas and his troops followed him to Egypt, whereupon Ptolemy conspired with the satrap of Media, Peithon, and the commander of the Argyraspides, Antigenes, both serving as officers under Perdiccas, and assassinated him.
Cornelius Nepos mentions that Seleucus also took part in this conspiracy, but this is not certain.
Alexander the Great died without a successor in Babylon on June 10, 323 BC. His general Perdiccas became the regent of all of Alexander's empire, while Alexander's physically and mentally disabled half-brother Arrhidaeus was chosen as the next king under the name Philip III of Macedon. Alexander's unborn child (Alexander IV) was also named his father's successor. In the "Partition of Babylon" however, Perdiccas effectively divided the enormous Macedonian dominion among Alexander's generals. Seleucus was chosen to command the Companion cavalry (hetairoi) and appointed first or court chiliarch, which made him the senior officer in the Royal Army after the regent and commander-in-chief Perdiccas. Several other powerful men supported Perdiccas, including Ptolemy, Lysimachus, Peithon and Eumenes. Perdiccas' power depended on his ability to hold Alexander's enormous empire together, and on whether he could force the satraps to obey him.
War soon broke out between Perdiccas and the other Diadochi. To cement his position, Perdiccas tried to marry Alexander's sister Cleopatra. The First War of the Diadochi began when Perdiccas sent Alexander's corpse to Macedonia for burial. Ptolemy however captured the body and took it to Alexandria. Perdiccas and his troops followed him to Egypt, whereupon Ptolemy conspired with the satrap of Media, Peithon, and the commander of the Argyraspides, Antigenes, both serving as officers under Perdiccas, and assassinated him. Cornelius Nepos mentions that Seleucus also took part in this conspiracy, but this is not certain.
Seleucus I Nicator