The Battle of the Oenoparus was a battle that took place in 145 BC on the Oenoparus river (the modern Afrin River, Syria) in the adjoining countryside of Antioch on the Orontes, the capital of the Seleucid Empire. It was fought between a coalition of Ptolemaic Egypt led by Ptolemy VI and Seleucids who favored the royal claim of Demetrius II Nicator against Seleucids who favored the claim of Alexander Balas. Both the Seleucid Empire and the Ptolemaic kingdom were diadochi, Greek-ruled successor states established after the conquests of Alexander the Great. The Ptolemaic force won the battle, and Balas's hopes of securing the throne were ended. However, King Ptolemy VI suffered a mortal wound in the battle. Despite the Egyptian forces winning the battle, they too would be driven out of Syria by the now unified Seleucid Empire under Demetrius II, which turned on the leaderless Egyptian force and drove them out of Seleucid territory. Background King Demetrius I Soter of the Seleucid Empire was ambitious and competent, but made enemies during his reign, both external and internal. The Roman Republic and the Kingdom of Pergamon, eager to weaken Demetrius and the Seleucids, backed a rival claimant to the Seleucid throne: Alexander Balas. Balas claimed to be a lost child of Antiochus IV Epiphanes, Demetrius I's uncle, and landed in the city of Ptolemais in 152 BC, backed by Roman and Pergamese funded mercenaries. He secured the aid of at least some internal rivals to Demetrius, notably including the Maccabees led by Jonathan Apphus, whom he appointed as high priest of Judea. Balas and his faction succeeded in killing Demetrius I in 150 BC and claimed the Seleucid throne for himself. Alexander Balas initially had the support of Ptolemaic Egypt and its king Ptolemy VI Philometor; he married Ptolemy's daughter Cleopatra Thea to create an alliance. However, this peace was not to last. Demetrius II, Demetrius I's young son, made a play to overthrow Balas, who gained a reputation as a weak and immoral ruler (whether deserved or not). The civil war resumed, and Egyptian forces massed on the border around 147 BC, ready to intervene in the Seleucid civil war. With Alexander's permission, the Egyptians occupied much of coastal Coele-Syria, with the cooperation of Alexander's Jewish allies who expanded and took over more of the Judean hills and interior. As Ptolemy VI marched north, he switched sides and