**Seleucus I Nicator** ("Seleucus the Victor") (c. 358 BC – 281 BC) was one of the generals friends of Alexander who were given, or took control of, his empire. Having served as an infantry general under Alexander, he eventually took the title of basileus (~"king"). He set up the Seleucid Empire over much of the territory in the  Near East which Alexander had conquered.

After the death of Alexander in June 323 BC, Seleucus initially supported Perdiccas, the regent of Alexander's empire, and was made Commander of the Companions (the elite cavalry of Alexander) at the Partition of Babylon in 323 BC.

The 'Wars of the Diadochi' in 322 were the wars of Alexander's successors. Perdiccas failed against Ptolemy in Egypt, which led to the mutiny of his troops. Perdiccas was betrayed and assassinated in a conspiracy by Seleucus and others sometime in either 321 or 320 BC.

In 321 BC, Seleucus was appointed Satrap of Babylon under the new regent Antipater. In 316/5 BC, Eumenes of Cardia entered his satrapy, trying to move to the east away from Antigonus. He stayed in Babylon and moved to Susa in the spring. After the defeat of Eumenes, Seleucus thought to flee Babylon because he feared for his life. Immediately in Spring 314BC, the wars between the Diadochi resumed and Antigonus with main operations in Coele-Syria. Seleucus was only able to return to Babylon in 312 BC with the support of Ptolemy with 50 horsemen accompanying him.

From 312 BC, Seleucus ruthlessly expanded his dominions and eventually conquered the Persian and Median lands because Antigonus was busy dealing with Cassander and Potolemy. Seleucus ruled not only Babylonia, but the entire enormous eastern part of Alexander's empire:

> "Always lying in wait for the neighbouring nations, strong in arms and persuasive in council, he [Seleucus] acquired Mesopotamia, Armenia, 'Seleucid' Cappadocia, Persis, Parthia, Bactria, Arabia, Tapouria, Sogdia, Arachosia, Hyrcania, and other adjacent peoples that had been subdued by Alexander, as far as the river Indus, so that the boundaries of his empire were the most extensive in Asia after that of Alexander. The whole region from Phrygia to the Indus was subject to Seleucus".
> 
> — Appian, The Syrian Wars

Seleucus' wars took him as far as India, where, after two years of war (305-303 BC), he made peace with the Indian Emperor Chandragupta Maurya. Seleucus exchanged his eastern satrapies in the Indus River Valley for 500 war elephants. 

  Seleucid Empire after war with Chandragupta Maurya
These would play a decisive role against Antigonus at the Battle of Ipsus in 301 BC and against Lysimachus at the Battle of Corupedium in 281 BC. After the battle, he gained the rest of Asia Minor except for Thracian Bithynia, Paphlagonia, Hreaclea Pontica, Caria and Pamphylia(under Ptolemies), Psidia(exception of Salagassos which was acquired during Antigonus’s rule), Lyconia, Rhodian Peraia, and Isauria. However, reigons like the Troad, Caria, and the islands had leagues which were vassals to the Seleucids by 281BC. Notably Halicarnassus, Pergamon, and Abydos.

Seleucus' victories against Antigonus and Lysimachus left the Seleucid dynasty virtually unopposed in Asia and in Anatolia. However, Seleucus also hoped to take control of Lysimachus' European territories, primarily Thrace and Macedon itself. But upon arriving in Thrace in 281 BC, Seleucus was assassinated by Ptolemy Ceraunus, who had taken refuge at the Seleucid court with his sister Lysandra. The assassination of Seleucus destroyed Seleucid prospects in Thrace and Macedon, and paved the way for Ptolemy Ceraunus to absorb much of Lysimachus' former power in Macedon. Seleucus was succeeded by his son Antiochus I as ruler of the Seleucid Empire.

Seleucus founded a number of new cities during his reign, including Antioch (300 BC) and in particular Seleucia on the Tigris (ca. 305 BC), the new capital of the Seleucid Empire, a foundation that eventually depopulated Babylon.