When they joined battle, it happened that a few of the Jews fell: They joined battle means the two armies met in battle. A few of the Jews fell means a few of the Jewish soldiers were killed in the battle. The reason why they were killed is explained by verses 39-40, where it is pointed out that all of the dead were guilty of idolatry. We suggest a model for this verse at the end of the end of the discussion on verse 35.

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.