Deuteronomy 9:3

Know, therefore, today that the Lord your God, whose glorious Shekinah goeth before you, whose Word is a consuming fire, will destroy them and drive them out before you; so shall you drive them out, and destroy them quickly, as the Lord your God hath said to you.

The metaphor a devouring fire connotes a rapid destruction. Thus, Scripture subsequently says quickly.

HE WILL DESTROY THEM, AND HE WILL BRING THEM DOWN. The meaning thereof is that “He will bring them down and He will destroy them” [because, obviously, they could be destroyed after having been brought down, but not vice versa]. The expression ‘yachni’eim’ (He will bring them down) denotes “humiliation,” as in the usage, Seest thou how Ahab ‘nichna’ (humbleth himself) before Me?; if then perchance their uncircumcised heart ‘yikana’ (be humbled), and so all [similar expressions]. And the purport thereof is that these peoples were very frightened of Israel. With a trembling heart, low spirit and feebleness of hands they went out [against them in battle], as it is stated, And it came to pass, when all the kings of the Amorites, that were beyond the Jordan westward, and all the kings of the Canaanites, that were by the sea, heard how the Eternal had dried up the waters of the Jordan etc. that their heart melted, neither was there spirit in them any more, because of the children of Israel. It is possible that the expression He will destroy them alludes to the nations greater and mightier than thyself — and He will bring them down refers to the sons of the Anakim, for despite all their might and height, their hearts were more humbled than [those of] the rest of the people and they did not go into battle at all, but were hiding in the mountains and in the fortified cities, as it is said, And Joshua came at that time, and cut off the Anakim from the hill-country, from Hebron, from Debir, from Anab, and from all the hill-country of Judah, and from all the hill-country of Israel; ‘with’ their cities Joshua utterly destroyed them, that is to say he destroyed them “in” their cities [indicating that the Anakim never entered the battlefield]. Similarly, it is said, And Judah went against the Canaanites that dwelt in Hebron etc. and they smote Sheshai, and Ahiman, and Talmai.

והאבדתם מהר, all the battles you will fight will result in your speedy victory, without your having to exert yourself, nor will you have to lay siege for a long period. In spite of all this, you will not be able to drive out all the inhabitants of the land in short order as the result would be a vacuum and the free roaming beasts would take over the part of the land which you could not populate immediately. (compare Deut. 7,22) This is the reason why I will not let you conduct wars immediately against all the Canaanite tribes. The words לא תוכל, are in the nature of a warning, just as in Deuteronomy 17,15 where the Torah warns against appointing a king who is not born of Jewish parents, or as in Deuteronomy 12,17 לא תוכל לאכול בשעריך, where the Torah forbids the eating in your homes instead of in Jerusalem of certain gifts vowed to G’d by you.