31 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 2 (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), 3 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. 4 And they were left, that He might test Israel by them, to know whether they would obey the commandments of the Lord, which He had 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.