John Gill

And it shall come to pass, that thy choicest valleys,.... The valleys that were near Jerusalem, that used to be covered with the choicest corn or vines, or with grass and flocks of sheep, and used to be exceeding delightful and pleasant: shall be full of chariots; where they can be more easily driven than on mountains; these were chariots not for pleasure, but for war; chariots full of soldiers, to fight against and besiege Jerusalem: and the horsemen shall set themselves in array at the gate: to take them that come out of the city, and to force their way into it; as well as to protect and defend the foot, while they made the assault, and scaled the walls, and to be ready when the gates were opened to them.