John Calvin Commentary Deuteronomy 31:4

"And Jehovah will do unto them as he did to Sihon and to Og, the kings of the Amorites, and unto their land; whom he destroyed." — Deuteronomy 31:4 (ASV)

And the Lord will do to them. He promises that, when they come into the land of Canaan, they will be conquerors of all its nations; and this He confirms by experience, for as God had delivered Sihon king of the Amorites and Og king of Bashan into their hands, so also He would give them the same success in subduing their other enemies. The world is indeed subject to many revolutions, but God still remains Himself, not only because His counsel is never changed, but also because His power is never diminished.

By a real proof, therefore, as it is called, He encourages the expectations of the people and at the same time exhorts them resolutely to execute God’s command, namely, that they should purge the land of Canaan by the destruction of all its inhabitants.

In appearance, indeed, this was fierce and cruel, to leave not even one alive. But since God had justly devoted them to extinction, it was not lawful for the Israelites to question what was to be done, but to abandon all discussion and obey God’s command. Because they spared many, their remissness was all the worse, since God had often prepared them to execute the vengeance He had decreed.