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.

As Ptolemy VI marched north, he switched sides and demanded his son-in-law Balas hand over his chief minister on likely faked charges.

Ptolemy VI now marched on Antioch; Alexander abandoned the city, apparently disliking his chances in a siege, and left north for Cilicia in Asia Minor.

Ptolemy VI had conquered the capital of Syria, with Demetrius II as a puppet ruler; his daughter Cleopatra Thea had her marriage with Balas annulled, and she was remarried to the new king.

Meanwhile, Balas left Cilicia and re-entered Syria; his supporters pillaged the countryside around Antioch to put pressure on the Egyptian invaders.

With the Seleucids briefly unified, Demetrius II turned on his former Egyptian allies and was able to banish the Ptolemaic occupation force out of Antioch and Coele-Syria.