John Gill Commentary Ezekiel 39:5

"Thou shalt fall upon the open field; for I have spoken it, saith the Lord Jehovah." — Ezekiel 39:5 (ASV)

You shall 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, says 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.