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 - due to the influx of wild beasts?
Rashi and Ramban (citing the Sifri): Moshe 1 is predicting here that Yisrael will sin, rendering them unworthy of the miracle 2 that no wild beasts will enter the towns that are temporarily devoid of inhabitants (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
See Pesukim 22-24.