If it seems like Moses is using superlative after superlative to talk about the incoming victories over Canaan, that’s because he is. There are no shortage of them; according to Deuteronomy 7, they shall “consume all peoples” and “no man will be able to stand before [Israel].” It is non-stop excitement.
Until you get to Deuteronomy 7:22, where God discusses the pace of the Canaanite expedition.
Lest Israel think they will come in and do this job quickly, Moses tells them that God will only clear out the nations in Canaan “little by little.” This conquest won’t take months, but years.
Why? Apparently, God knew that if they achieved total victory in a short period of time, the population rate wouldn’t be able to keep up. They literally wouldn’t have enough people to fill the land—to the point that wild animals would pose a real threat to the new inhabitants.
This shouldn’t scare the people. In fact, the atypical conquest would cause the actual intended effect, which is to completely blot out even the memory of these types of people.
Think about it. If Israel moved quickly through the land, they wouldn’t have the opportunity to burn all the graven images with fire (Deuteronomy 7:25). Only by advancing slowly will they be able to completely raze these cities to the ground.
While they’re doing that, God will be busy causing chaos on the other end.
The mention of hornets in Deuteronomy 7:20 is particularly interesting, primarily because there is no mention of actual bees stinging the Canaanites. The language seems to invoke that of the insect plagues in Egypt, but if so, there is no mention of this actually happening.
What does God mean, then? If there are no hornets, then how did God drive them out before Israel?
It’s not known (by me, at least) what exactly is referred to here, but we do know that God kept this promise. In Joshua 24:12, God tells Israel that He “sent the hornet” against Sihon and Og before they even entered Canaan, so whatever it is, Israel knew what God meant.
Regardless, a prolonged campaign by Israel was the best option because it allowed Israelite numbers to grow, while also neutralizing any reminder of the idolatry in the land. Once those were taken care of, then the land was free for the taking.