John Calvin Commentary Numbers 24:21

"And he looked on the Kenite, and took up his parable, and said, Strong is thy dwelling-place, And thy nest is set in the rock." — Numbers 24:21 (ASV)

And he looked on the Kenites. I have not yet referred to the sense in which Balaam is said to have seen the Kenites, as well as the other nations. And now, I should also refrain from doing so, if some did not attribute it to prophetic vision, an opinion with which I cannot agree. For Moses relates as a matter of history that Balaam turned his face in the directions in which they respectively lived; and, although he did not actually see the people themselves, the sight of the place in which they lived was sufficient for the purpose of prophecy.

By the Kenites I understand the Midianites, who were adjacent to the Amalekites, as it is altogether unreasonable to refer the name to the descendants of Jethro. Forty years had not yet elapsed since Jethro had left his son with Moses; and his was only one small family in the wilderness of Midian, whereas this passage refers to an already celebrated people.

Balaam, therefore, designates the Midianites by synecdoche and also devotes them to the punishment they well deserved. Gideon was, in some measure, the minister and executioner of this when he routed their immense army with three hundred men; his victory is celebrated in Psalm 83:11 and Isaiah 9:4. It is probable that their power was broken at that time.