Demetrius III Theos Philopator Soter Philometor Euergetes Callinicus, surnamed Eucaerus; between 124 and 109 BC – after 87 BC) was a Hellenistic Seleucid monarch who reigned as the King of Syria between 96 and 87 BC. He was a son of Antiochus VIII and, most likely, his Egyptian wife Tryphaena. Demetrius III’s early life was spent in a period of civil war between his father and his uncle Antiochus IX, which ended with the assassination of Antiochus VIII in 96 BC. After the death of their father, Demetrius III took control of Damascus while his brother Seleucus VI prepared for war against Antiochus IX, who occupied the Syrian capital Antioch.
The civil war dragged on, and Seleucus VI eliminated his uncle, whose heir Antiochus X counterattacked and drove Seleucus VI to his death. Then the twins Antiochus XI and Philip I, brothers of Demetrius III, attempted to avenge Seleucus VI but it ended with the death of Antiochus XI and the interference of Demetrius III on the side of Philip I in a war against Antiochus X that probably lasted until 88 BC. In 89 BC, Demetrius III invaded Judaea and crushed the forces of its king, Alexander Jannaeus and his near victory was cut short by the death of Antiochus X. Demetrius III rushed to Antioch before Philip I could take advantage of the power vacuum and strengthen his position relative to Demetrius III.
By 87 BC, Demetrius III had most of Syria under his authority. He attempted to appease the public by promoting the importance of the local Semitic gods, and he might have given Damascus the dynastic name Demetrias. By late 87 BC, Demetrius III attacked Philip I in the city of Beroea, where Philip I’s allies called on the Parthians for help. The allied forces routed Demetrius III and besieged him in his camp; he was forced to surrender and spent the rest of his life in exile in Parthia. Philip I took Antioch, while Antiochus XII, another brother of Demetrius III, took Damascus.