Judges 3

Judges 3:1-2:
Now these are the nations that the LORD left, to test Israel by them, that is,
all in Israel who had not experienced all the wars in Canaan. It was in order that the generations of the people of Israel might know war, to teach war to those who had not known it before

In one sense, as the book of Judges pounds into our heads over and over, the enemy nations are there because Israel didn’t believe God enough to drive them out. But that’s not what Judges 3 says. No, here we see that God left them there to test Israel so that they might learn to fight wars.
God wanted to give Israel the land of Canaan. But apparently, he wanted to do it through struggle. So he continued to test them, to see if they would believe him, to teach them to trust him in their fight. He does the same with us, though (as Paul reminds us) our battle isn’t against flesh and blood, but against spiritual powers. Why doesn’t God remove our struggles when we become Christians? Because he wants us to keep relying on his grace, not on our flesh. As Paul said, some of the weaknesses and trials in our lives are there—by design—to keep us humble. ”
--JD Grear

Judges 3:10:
When all that generation had been gathered to their fathers, another generation arose after them who did not know the Lord nor the work which He had done for Israel

Then in Judges 3:1-2 we read, “Now these are the nations which the Lord left, that He might test Israel by them, that is, all who had not known any of the wars in Canaan (this was only so that the generations of the children of Israel might be taught to know war, at least those who had not formerly known it)…” The key here is not that this generation did not know how to fight. It is that they did not know the cost that had been paid for them to possess the Land. This next generation had a mentality of entitlement. They inherited a land that had been purchased for them.
The people who had lived through the Conquest and had seen the power and works of God failed to teach these things to their children. Listen to the warning and instruction on Deut. 6:4 “Hear, O Israel: The Lord our God, the Lord is one! 5 You shall love the Lord your God with all your heart, with all your soul, and with all your strength. 6 “And these words which I command you today shall be in your heart. 7 You shall teach them diligently to your children, and shall talk of them when you sit in your house, when you walk by the way, when you lie down, and when you rise up. 8 You shall bind them as a sign on your hand, and they shall be as frontlets between your eyes. 9 You shall write them on the doorposts of your house and on your gates. 10 “So it shall be, when the Lord your God brings you into the land of which He swore to your fathers, to Abraham, Isaac, and Jacob, to give you large and beautiful cities which you did not build, 11 houses full of all good things, which you did not fill, hewn-out wells which you did not dig, vineyards and olive trees which you did not plant—when you have eaten and are full— 12 then beware, lest you forget the Lord who brought you out of the land of Egypt, from the house of bondage.
They forgot God. They did not know the Lord because they had not been reminded of all that He had done to preserve and protect them as a nation. They did not know war, that is, the cost of the conquest. ”
-Time in the Word Ministries