Isaiah 42:15

I will make waste mountains and hills,.... Kingdoms, greater and lesser; kings and governors, as Jarchi interprets it; and so Kimchi understands it of the kings of the nations; by them are meant the emperors of Rome, and their governors under them, that set themselves against Christ and his Gospel, but were overcome by him; these mountains and hills became a plain before him: "every mountain and island were moved out of their places, and the kings of the earth, and the great men, c. hid themselves in the dens, and in the rocks of the mountains, and called upon them to fall on them, and hide them from the wrath of the Lamb", Revelation 6:14:

and dry up all their herbs the common people, and common soldiers that were with them, and on their side; comparable, for smallness, weakness, and number, to the grass of the mountains and hills:

and I will make the rivers islands, and dry up the pools; extirpate all the remains of idolatry, rivers and fountains being sacred with the Heathens, as mountains and hills were places where sacrifices were offered to idols. Unless by it rather should be meant, that the Lord would remove all impediments out of the way of his people, or which were obstacles of their conversion; just as he dried up the waters of the Red sea and Jordan, to make way for the people of Israel; to which the allusion may be, and which agrees with the following words.