31 Now these are the nations that the Lord left to test all those in Israel who had no experience of any war in Canaan 2 (it was only that successive generations of Israelites might know war, to teach those who had no experience of it before): 3 the five lords of the Philistines, and all the Canaanites, and the Sidonians, and the Hivites who lived on Mount Lebanon, from Mount Baal-hermon as far as Lebo-hamath. 4 They were for the testing of Israel, to know whether Israel would obey the commandments of the Lord, which he commanded their ancestors by 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.