All the troops that Demetrius had cast off gathered around him: The pronoun him refers to the young King Antiochus. We may also express this clause as “All the soldiers that King Demetrius~II had sent home now sided with [or, came to the support of] Antiochus.”

And they fought against Demetrius, and he fled and was routed: They fought against Demetrius may be rendered “they attacked Demetrius.” For he fled and was routed, Good News Bible says “They defeated Demetrius, and he fled,” and Contemporary English Version has “forcing him to run for his life.” Demetrius was apparently still in the palace in Antioch when Trypho made his move. So Demetrius fled Antioch, retreating to either Cilicia (Josephus) or Seleucia (Livy).

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.