Ezekiel 39:5

Thou shalt fall upon the open field
Some part of his army should fall upon the mountains, and others upon the plain; wherever they will be found, they will be destroyed, either by the sword of the Jews and Christian princes, or by God's judgments from heaven: for I have spoken it, saith the Lord God;
and therefore it should surely come to pass, since no word of his ever fails; this is added to assure of the truth of it, since it might be thought incredible that so large an army should be destroyed.