The war elephants Seleucus received from Chandragupta proved to be useful when the Diadochi finally decided to deal with Antigonus.
Cassander, Seleucus and Lysimachus defeated Antigonus and Demetrius in the battle of Ipsus.
Antigonus fell in battle, but Demetrius escaped.
After the battle, Syria was placed under Seleucus' rule.
He understood Syria to encompass the region from the Taurus mountains to Sinai, but Ptolemy had already conquered Palestine and Phoenicia.
In 299 BC, Seleucus allied with Demetrius and married his daughter Stratonice.
Stratonice was also the daughter of Antipater's daughter Phila.
Seleucus had a daughter by Stratonice, who was also called Phila.

The fleet of Demetrius destroyed Ptolemy's fleet and thus Seleucus did not need to fight him.

Seleucus, however, did not manage to enlarge his kingdom to the west.
The main reason was that he did not have enough Greek and Macedonian troops.
During the battle of Ipsus, he had less infantry than Lysimachus.
His strength was in his war elephants and in traditional Persian cavalry.
In order to enlarge his army, Seleucus tried to attract colonists from mainland Greece by founding four new cities—Seleucia Pieria and Laodicea in Syria on the coast and Antioch on the Orontes and Apameia in the Orontes River valley.
Antioch became his chief seat of government.
The new Seleucia was supposed to become his new naval base and a gateway to the Mediterranean.
Seleucus also founded six smaller cities.

It is said of Seleucus that "few princes have ever lived with so great a passion for the building of cities.
He is reputed to have built in all nine Seleucias, sixteen Antiochs, and six Laodiceas".
The war elephants Seleucus received from Chandragupta proved to be useful when the Diadochi finally decided to deal with Antigonus. Cassander, Seleucus and Lysimachus defeated Antigonus and Demetrius in the battle of Ipsus. Antigonus fell in battle, but Demetrius escaped. After the battle, Syria was placed under Seleucus' rule. He understood Syria to encompass the region from the Taurus mountains to Sinai, but Ptolemy had already conquered Palestine and Phoenicia. In 299 BC, Seleucus allied with Demetrius and married his daughter Stratonice. Stratonice was also the daughter of Antipater's daughter Phila. Seleucus had a daughter by Stratonice, who was also called Phila.
The fleet of Demetrius destroyed Ptolemy's fleet and thus Seleucus did not need to fight him.
Seleucus, however, did not manage to enlarge his kingdom to the west. The main reason was that he did not have enough Greek and Macedonian troops. During the battle of Ipsus, he had less infantry than Lysimachus. His strength was in his war elephants and in traditional Persian cavalry. In order to enlarge his army, Seleucus tried to attract colonists from mainland Greece by founding four new cities—Seleucia Pieria and Laodicea in Syria on the coast and Antioch on the Orontes and Apameia in the Orontes River valley. Antioch became his chief seat of government. The new Seleucia was supposed to become his new naval base and a gateway to the Mediterranean. Seleucus also founded six smaller cities.
It is said of Seleucus that "few princes have ever lived with so great a passion for the building of cities. He is reputed to have built in all nine Seleucias, sixteen Antiochs, and six Laodiceas".
Seleucus I Nicator