Seeing as, when Yisrael do the will of Hakadosh-Baruch-Hu, the wild animals will do them no harm (See Iyov, 5:53), why does the Torah inform them that they will not be able to destroy the Cana'ani nations quickly on account of the wild beasts?
Rashi (citing the Sifri) and Ramban: Moshe 1 is predicting here that Yisrael will sin, rendering them unworthy of the miracle 2 that wild beasts will not harm them (when they come to devour the carcasses of the slain - Targum Yonasan).
What are the implications of the word "ve'ha'Nish'arim"?
Ramban: It implies that the hornets will only be needed to ferret out the small minority of refugees who will go into hiding after the vast majority of the seven nations, including all of the thirty-one kings, have fallen in battle, their status as nations destroyed forever. 1
QUESTIONS ON RASHI
Rashi writes that Yisrael will sin, and not merit that wild beasts not harm them. If so, also the remaining Kena'anim (since you cannot destroy them quickly) will be able to harm them!
Moshav Zekenim: The Kena'anim cannot harm them, for they are bigger sinners