31 And these 'are' the nations which Jehovah left, to try Israel by them, all who have not known all the wars of Canaan; 2 (only for the sake of the generations of the sons of Israel's knowing, to teach them war, only those who formerly have not known them)— 3 five princes of the Philistines, and all the Canaanite, and the Zidonian, and the Hivite inhabiting mount Lebanon, from mount Baal-Hermon unto the entering in of Hamath; 4 and they are to prove Israel by them, to know whether they obey the commands of Jehovah that He commanded their fathers by the hand of Moses.
Matthew Henry's Commentary on Judges 3:1-4
Commentary on Judges 3:1-7
(Read Judges 3:1-7)
As the Israelites were a type of the church on earth, they were not to be idle and slothful. The Lord was pleased to try them by the remains of the devoted nations they spared. Temptations and trials detect the wickedness of the hearts of sinners; and strengthen he graces of believers in their daily conflict with Satan, sin, and this evil world. They must live in this world, but they are not of it, and are forbidden to conform to it. This marks the difference between the followers of Christ and mere professors. The friendship of the world is more fatal than its enmity; the latter can only kill the body, but the former murders many precious souls.