Komentarz do Sędziów 3:2

Only so that they should know. For this alone, He permitted these nations to remain, so that future generations of the Bnei Yisroel might grasp the consequence of iniquity—for it was now necessary for them to undergo martial training.

ONLY SO THAT THEY SHOULD KNOW: It wants to teach that only because of this they will become aware of the wonders of the miracles that were done for them since then

Whereas earlier when they stood worshiping Adonoy.

...TO TEACH THEM WAR: It wants to say that with the remaining nations, did (Israel) not fight them (the nations)? Therefore it was a need to teach them the strategies of war. Because the generation before them, in the days of Yehoshua, they (that generation) did not know the strategies of war because Hashem fought for them. But after that, they needed to know it because of their actions Hashem will leave them to chance. Therefore they should be informed to know the miracles that were done since and trust in Hashem from now on

This was unknown. These wars and the art of battle. It was unnecessary.