John Calvin Commentary Deuteronomy 7:20

"Moreover Jehovah thy God will send the hornet among them, until they that are left, and hide themselves, perish from before thee." — Deuteronomy 7:20 (ASV)

Moreover, the Lord thy God will send the hornet. Since the destruction of their enemies might seem long if they were only to be slain by their hands and weapons, and also because it was hardly believable that, without defending themselves, they would voluntarily offer their own throats, God promises that He would also supply the means of their conquest in another way. Therefore, to prevent the Israelites from being alarmed or frightened by imagining that their enemies would be prompt and vigorous in resistance, God declares that other forces would be available, because hornets or other poisonous insects would destroy all the fugitives.

The same declaration is found in Exodus 23; and Joshua relates that God performed what He had promised (Joshua 24:12). But since these nations were not to be destroyed in a moment, so that the people would not therefore grow weary or become inactive, God anticipates this and reminds them that this delay would be advantageous, because when all the inhabitants were exterminated, wild beasts would occupy the empty land. The prolongation of the war, therefore, should not trouble them, for by it God provided for His people’s welfare, since, if the men were quickly destroyed, they would have to contend with wild beasts.

But though the passage I have quoted from Exodus is similar in terms, I have intentionally placed it under another heading; for God here refers to the extermination of the Gentile nations with another object: that is, so that no ancient pollutions would remain in the land, and so that the Israelites would not mix with the ungodly, by whose schemes they might eventually be led astray to false religions.