Only that the generations of the children of Israel might know
and teach them war
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, that they might be trained up in the knowledge of warlike affairs, and so be also capable of teaching their children the military art; which they should make use of in obeying the command of God, by driving out the remains of the Canaanites, and not give themselves up to sloth and indolence; though some think that the meaning is, that God left these nations among them, that they might know what war was, and the sad effects of it; and the difference of fighting with their enemies alone, as other men, and the Lord fighting along with them, and for them, as he did for their fathers:
at least such as before knew nothing thereof;
being either unborn, or at an age incapable of bearing arms, or learning the art of war.