What does Judges 20:39 mean?
Israel's renewed battle plan has worked out extremely well (Judges 20:29–31). Instead of continuing to assail the city of Gibeah by direct attack (Judges 20:20–28), the Israelites have drawn Benjamin's army out from the city with a false retreat. When they were far enough away, a separate contingent of Israel's army attacked and defeated the city, killing all inside (Judges 20:29–38).As Gibeah falls, the soldiers of Benjamin are confident they are routing the Israelites: chasing them down to finish them off. This verse repeats the previous statement that Benjamin has only killed about thirty Israelite fighters in this chase, adding to the illusion they have them on the run. The Benjaminites assume this battle is going the same way as the previous two and will end once again in Israel's defeat.
The retreating Israelites, however, are just waiting until they see the smoke of Gibeah's defeat appear in the sky. That's the designated signal to halt the false retreat, turn, and strike down Benjamin's army. When that smoke appears, even the Benjaminite army will realize what has happened (Judges 20:40–41).