New Revised Standard Version
the main body of Israel should turn in battle. But Benjamin had begun to inflict casualties on the Israelites, killing about thirty of them; so they thought, “Surely they are defeated before us, as in the first battle.”

New American Bible
and the men of Israel would then wheel about in the battle. Benjamin, having begun by killing off some thirty of the men of Israel, thought, “Surely they are completely routed before us, as in the earlier fighting.”

Douay-Rheims Bible
And when the children of Israel saw this in the battle, (for the children of Benjamin thought they fled, and pursued them vigorously, killing thirty men of their army)

And when the children of Israel saw this in the battle, (for the children of Benjamin thought they fled, and pursued them vigorously, killing thirty men of their army)

And when

Judges 20:31 And the children of Benjamin boldly issued out of the city, and seeing their enemies flee, pursued them a long way, so as to wound and kill some of them, as they had done the first and second day, whilst they fled by two highways, whereof one goeth up to Bethel and the other to Gabaa, and they slew about thirty men: