"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
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(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),
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namely, five lords of the Philistines, all the Canaanites, the Sidonians, and the Hivites who dwelt in Mount Lebanon, from Mount Baal Hermon to the entrance of Hamath."
Judges 3:1-3
When the people of Israel neglected to drive out the inhabitants of the land according to God's command, He allowed the people to remain to test the Israelites. In Judges 2:22 God revealed one purpose why He allowed the nations to remain: "...So that through them I may test Israel, whether they will keep the ways of the LORD, to walk in them as their fathers kept them, or not." By allowing nations that did not fear God to remain, He effectively provided His people a choice. The Israelites were granted an opportunity to prove themselves faithful to God's commands or reveal their disobedience.
This second reason God allowed the nations who did not fear God among His people I find most compelling. There was a new generation of Israelites who did not know God, His mighty works (Judges 2:10), and had not experienced war firsthand. They were not battle-hardened warriors who placed their faith in God to fight their battles. This new generation was soft, easily influenced by the enemies of God, and needed to learn how to fight and stand up for righteousness. So God saw fit to allow enemies of Israel to remain so His people would "be taught to know war." No one gains skill through ignorance. No one improves without a level of training and practical experience.
It is for this reason that God has allowed Satan - the enemy of our souls - to remain presently alive and well on this earth. He goes to and fro like a hungry lion, seeking who he may devour. We are called to resist him, steadfast in the faith, standing strong girded with armour supplied through the Holy Spirit. God wants Christians to learn how to fight, defend the truth, to resist temptation, and to be strong. God does not want us to be warmongers against flesh and blood, but to be battle tested and grow increasingly proficient with our blade: the Sword of the Spirit which is is the Word of God. He wants our minds to be sharp, our lives focused, our hands toughened, taking courage in God though we see the enemy advancing. No one in the midst of a fight entangles himself with the affairs of this life, and we must remain vigilant. God wants us to be battle-hardened veterans, not doughy and soft through lack of exercise. He desires we be courageous and bold to trust His promises. Every day we are being tested and tried: are we willing to make peace with the world through compromise? Are we willing to stand up for righteousness or will we acquiesce to the world's wisdom to please the flesh? Will we choose to love God and others enough to labour to pull down the enemy's strongholds?
God has provided all His people with a choice. Our desires, decisions, and the use of our time shows if we are truly loyal to God or not. May God be able to say to us at the end of our days: "Well done! You have fought the good fight; you have run a good race; you have finished the course. Enter into the joy of the LORD." Even as the enemy gains strength, we need not fear. Greater is He that is in us than he that is in the world. The battle is the LORD's, but we must be resolute to fight!