Judges 3:2

Teach them war. The ones who belonged to the new generation of Israel did not know the horrors of war, nor were they personally acquainted with the mighty deliverances that God had wrought for their forefathers. Having grown up in comparative ease, they turned their backs upon the God in whom their fathers had trusted for deliverance from a numerically superior enemy. God purposed through the nations that remained in and around Canaan, to repeat the lessons of His former mighty deliverances and of the impotency of heathen gods. By the wars that ensued, the younger generation of Israelites learned from bitter experiences that they could fight and conquer these numerous and warlike people only with the aid of the God of their fathers.