Now these are the nations which the LORD left, to prove Israel by them, even as many of Israel as had not known all the {a} wars of Canaan;

(a) Which were achieved by the hand of God, and not by the power of man.

Wesley's Notes

3:1 Had not known - That is, such as had no experience of those wars, nor of God's extraordinary power and providence manifested in them.

Jamieson-Fausset-Brown Bible Commentary

CHAPTER 3

Jud 3:1-4. Nations Left to Prove Israel.

1. these are the nations which the Lord left, to prove Israel-This was the special design of these nations being left, and it evinces the direct influence of the theocracy under which the Israelites were placed. These nations were left for a double purpose: in the first instance, to be instrumental, by their inroads, in promoting the moral and spiritual discipline of the Israelites; and also to subserve the design of making them acquainted with war, in order that the young, more especially, who were total strangers to it, might learn the use of weapons and the art of wielding them.

Matthew Henry's Concise Commentary

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.