They pursued them down the descent of Beth-horon to the plain: The Revised Standard Version footnote here indicates that some Greek manuscripts read “him” instead of them. The evidence for either reading is equally balanced, but we recommend following Revised Standard Version and Good News Bible with “them.” The descent of Beth-horon is the same as the “ascent of Beth-horon” in verse 16 (see the comments there). It is called descent here because Judas and his men moved down the pass. In some languages plain will be expressed as “flat level area.” A possible model for this whole clause is “They pursued them down the steep narrow path at Beth Horon to the plain.”

Eight hundred of them fell may be expressed as “Eight hundred enemy soldiers were killed” (Contemporary English Version) or “They killed about 800 men.”

And the rest fled into the land of the Philistines: The Philistines disappeared from history with the Babylonian conquests of the seventh century b.c. At the time of the Maccabees only the name remained, so the land of the Philistines means “the land where the Philistines used to live long ago.” This land was in the southwestern part of the country of Judea in the broad coastal plain. This whole clause may be rendered “Those who were not killed fled to the southwest.”

An alternative model for this verse is:

• Judas and his soldiers pursued them down the steep narrow path at Beth Horon to the plain [or, flat area below]. They killed about 800 [or, eight hundred] enemy soldiers. Those who were not killed [or, The survivors] escaped to the southwest.

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.