Adam Clarke on Judges 3:2

That - Israel might know, to teach them war - This was another reason why the Canaanites were left in the land, that the Israelites might not forget military discipline, but habituate themselves to the use of arms, that they might always be able to defend themselves against their foes. Had they been faithful to God, they would have had no need of learning the art of war; but now arms became a sort of necessary substitute for that spiritual strength which had departed from them. Thus Gods in his judgments leaves one iniquitous nation to harass and torment another. Were all to turn to God, men need learn war no more.