Charles Ellicott Commentary Judges 3:2

"only that the generations of the children of Israel might know, to teach them war, at the least such as beforetime knew nothing thereof:" — Judges 3:2 (ASV)

Only that the generations of the children of Israel might know, to teach them war. —The Septuagint here renders, “Only because of the generations of the children of Israel to teach them war.” The Vulgate is here a mere paraphrase, and the translations vary. The meaning seems to be, “Only that He (Jehovah) might know the generations of the children of Israel, to teach them war.” The expression resembles 2 Chronicles 32:31. The phrase “teaching them war” doubtless implies the lesson that they could only learn successfully by the help of God.

As before knew nothing thereof. —That is, “knew nothing of those nations, or of those wars.”