Judges 3:2

“Only that the generations of the children of Israel might know and teach them war, etc.] That is, the following nations were left in the land, that the young generations of Israel might by their wars and conflicts with them learn the art of war, and be inured to martial discipline; which, if none had been left to engage with, they had been ignorant of: besides, their fathers in Joshua’s time, as Jarchi and Kimchi observe, had no need to learn the art of war, for God fought for them; they did not get possession of the land by their own arm, and by their sword, but by the power of God in a miraculous way; but now this was not to be expected, and the Canaanites were left among them to expel,…”

“Teach them war - That by the neighbourhood of such warlike enemies, they might be purged from sloth and security, and obliged them to innure themselves to martial exercises, and to stand continually upon their guard, and consequently to keep close to that God whose assistance they had so great and constant need of. Judges 3:3”

“might know: Gen 2:17, Gen 3:5, Gen 3:7; Ch2 12:8; Mat 10:34-39; Joh 16:33; Co1 9:26, Co1 9:27; Eph 6:11-18; Ti1 6:12; Ti2 2:3, Ti2 4:7 to teach: Their fathers fought by a divine power. God taught their hands to war and their fingers to fight, that they might be the instruments of destruction to the wicked nations on whom the curse rested; but now that they had forfeited His favour, they must learn what it is to fight like other men. Judges 3:3”