Albert Barnes Commentary Deuteronomy 9:3

"Know therefore this day, that Jehovah thy God is he who goeth over before thee as a devouring fire; he will destroy them, and he will bring them down before thee: so shalt thou drive them out, and make them to perish quickly, as Jehovah hath spoken unto thee." — Deuteronomy 9:3 (ASV)

So shalt thou drive them out, and destroy them quickly - This is not inconsistent with (Deuteronomy 7:22), in which instant annihilation is not to be expected for the reasons assigned. Here Moses urges the people to trust in God’s covenanted aid; since He would then make no delay in so destroying the nations they attacked as to put them into enjoyment of the promises, and doing so as quickly as was for the well-being of Israel itself.