39 and then the Israelites would counterattack. The Benjamites had begun to inflict casualties on the Israelites (about thirty), and they said, “We are defeating them as in the first battle.”

Read Judges 20:39 Using Other Translations

And when the men of Israel retired in the battle, Benjamin began to smite and kill of the men of Israel about thirty persons: for they said, Surely they are smitten down before us, as in the first battle.
the men of Israel should turn in battle. Now Benjamin had begun to strike and kill about thirty men of Israel. They said, "Surely they are defeated before us, as in the first battle."
When the Israelites saw the smoke, they turned and attacked Benjamin’s warriors. By that time Benjamin’s warriors had killed about thirty Israelites, and they shouted, “We’re defeating them as we did in the first battle!”