Albert Barnes Commentary Judges 20:9

"But now this is the thing which we will do to Gibeah: [we will go up] against it by lot;" — Judges 20:9 (ASV)

By lot - To determine who should go up first (Judges 20:18). The shape of the ground probably made it impossible for the whole force to operate at once; and the question of spoil would have something to do with the arrangement. (Compare to 1 Samuel 30:22–25.)