if only to teach warfare to the subsequent generations of Israel, especially to those who had not known it formerly:
Their presence also serves to teach warfare to later generations of Israel who have not known it before.
It adds a second purpose for the nations being left in place.
1These are the nations that the LORD left to test all the Israelites who had not known any of the wars in Canaan, 2if only to teach warfare to the subsequent generations of Israel, especially to those who had not known it formerly: 3the five rulers of the Philistines, all the Canaanites, the Sidonians, and the Hivites who lived in the mountains of Lebanon from Mount Baal-hermon to Lebo-hamath. 4These nations were left to test the Israelites, to find out whether they would keep the commandments of the LORD, which He had given their fathers through Moses. 5Thus the Israelites continued to live among the Canaanites, Hittites, Amorites, Perizzites, Hivites, and Jebusites. 6And they took the daughters of these people in marriage, gave their own daughters to their sons, and served their gods.