John Gill Bible Commentary

And the Lord was with Judah,.... Encouraging, strengthening, succeeding, and giving the tribe victory over the Canaanites; the Targum is,"the Word of the Lord was for the help of the house of Judah:" and he drove out the inhabitants of the mountains; the mountainous part of Judea, such as was about Jerusalem, and where Hebron stood, and other cities, see Jos 15:48, &c. which though fortified both by nature and man, yet God being with them, they were easily subdued: but could not drive out the inhabitants of the valley; God forsaking them, because they were afraid of them, for a reason after mentioned, or through slothfulness, and being weary of fighting, or because they fell into some sins, which occasioned the divine displeasure; so the Targum,"after they had sinned, they could not drive out the inhabitants of the valley:" because they had chariots of iron; but this was no reason why they could not drive them out, if God was with them, who could as easily have delivered these into their hands, as the inhabitants of the mountains; but is the reason why they were afraid to fight with them, and to attempt to drive them out, and which they themselves gave why they did not.