Seleucus I Nicator (/səˈljuːkəs naɪˈkeɪtər/; c. 358 BC – September 281 BC; Ancient Greek: Σέλευκος Νικάτωρ, romanized: Séleukos Nikátōr, lit.
 'Seleucus the Victor') was one of the Diadochi (the rival generals, relatives, and friends of Alexander the Great who fought for control over his empire after his death).
Having previously served as an infantry general under Alexander the Great, he eventually assumed the title of basileus and established the Seleucid Empire over the bulk of the territory which Alexander had conquered in Asia.

After the death of Alexander in June 323 BC, Seleucus initially supported Perdiccas, the regent of Alexander's empire, and was appointed Commander of the Companions and chiliarch at the Partition of Babylon in 323 BC.
However, after the outbreak of the Wars of the Diadochi in 322, Perdiccas' military failures against Ptolemy in Egypt led to the mutiny of his troops in Pelusium.
Perdiccas was betrayed and assassinated in a conspiracy by Seleucus, Peithon and Antigenes in Pelusium sometime in either 321 or 320 BC.
At the Partition of Triparadisus in 321 BC, Seleucus was appointed Satrap of Babylon under the new regent Antipater.
But almost immediately, the wars between the Diadochi resumed and Antigonus forced Seleucus to flee Babylon.
Seleucus was only able to return to Babylon in 312 BC with the support of Ptolemy.
From 312 BC, Seleucus ruthlessly expanded his dominions and eventually conquered the Persian and Median lands.
Seleucus ruled not only  Babylonia, but the entire enormous eastern part of Alexander's empire.

Seleucus further made claim to the former satraps in Gandhara and in the Indus.
However these ambitions were contested by Chandragupta Maurya, resulting in the Seleucid–Mauryan War (305–303 BC).
The conflict was ultimately resolved by a treaty resulting in the Maurya Empire annexing the eastern satraps.
Additionally, a marriage alliance between the two empires was formalized with Chandragupta marrying the Seleucid Princess Helena.
Furthermore, the Seleucid Empire received a considerable military force of 500 war elephants with mahouts, which 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.

Seleucus' victories against Antigonus and Lysimachus left the Seleucid dynasty virtually unopposed amongst the Diadochi.
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 (c. 305 BC), the new capital of the Seleucid Empire, a foundation that eventually depopulated Babylon.
Seleucus I Nicator (/səˈljuːkəs naɪˈkeɪtər/; c. 358 BC – September 281 BC; Ancient Greek: Σέλευκος Νικάτωρ, romanized: Séleukos Nikátōr, lit. 'Seleucus the Victor') was one of the Diadochi (the rival generals, relatives, and friends of Alexander the Great who fought for control over his empire after his death). Having previously served as an infantry general under Alexander the Great, he eventually assumed the title of basileus and established the Seleucid Empire over the bulk of the territory which Alexander had conquered in Asia.
After the death of Alexander in June 323 BC, Seleucus initially supported Perdiccas, the regent of Alexander's empire, and was appointed Commander of the Companions and chiliarch at the Partition of Babylon in 323 BC. However, after the outbreak of the Wars of the Diadochi in 322, Perdiccas' military failures against Ptolemy in Egypt led to the mutiny of his troops in Pelusium. Perdiccas was betrayed and assassinated in a conspiracy by Seleucus, Peithon and Antigenes in Pelusium sometime in either 321 or 320 BC. At the Partition of Triparadisus in 321 BC, Seleucus was appointed Satrap of Babylon under the new regent Antipater. But almost immediately, the wars between the Diadochi resumed and Antigonus forced Seleucus to flee Babylon. Seleucus was only able to return to Babylon in 312 BC with the support of Ptolemy. From 312 BC, Seleucus ruthlessly expanded his dominions and eventually conquered the Persian and Median lands. Seleucus ruled not only  Babylonia, but the entire enormous eastern part of Alexander's empire.
Seleucus further made claim to the former satraps in Gandhara and in the Indus. However these ambitions were contested by Chandragupta Maurya, resulting in the Seleucid–Mauryan War (305–303 BC). The conflict was ultimately resolved by a treaty resulting in the Maurya Empire annexing the eastern satraps. Additionally, a marriage alliance between the two empires was formalized with Chandragupta marrying the Seleucid Princess Helena. Furthermore, the Seleucid Empire received a considerable military force of 500 war elephants with mahouts, which 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. 
Seleucus' victories against Antigonus and Lysimachus left the Seleucid dynasty virtually unopposed amongst the Diadochi. 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 (c. 305 BC), the new capital of the Seleucid Empire, a foundation that eventually depopulated Babylon.
Seleucus I Nicator