Deuteronomy 7:21-23

Thou shalt not fear them, because the Lord thy God is in the midst of thee, a God mighty and terrible:

He will consume these nations in thy sight by little and little and by degrees. Thou wilt not be able to destroy them altogether: lest perhaps the beasts of the earth should increase upon thee.

But the Lord thy God shall deliver them in thy sight: and shall slay them until they be utterly destroyed.