Moses continued his historical account of Israel in the wilderness. Israel left its 38-year cycle of circling in the desert after everyone of the first generation died. As Israel approached Canaan God forbade their attacking Edom, Moab, and Ammon because of their relation to them. Indeed, Israel was to pay these countries for any food or water they used.
As Israel made their way through those countries God caused the various Canaanite nations to be absolutely terrified of Israel. Consequently, though Moses peacefully asked permission of Sihon to pass through their land, Sihon absolutely refused and came out to destroy Israel. Unlike the account in Numbers which simply relates the historical details, here Moses provides the fuller perspective. Despite the terror at the coming Israelites (v. 25), Sihon and his people were bound and determined to fight Israel to the very last of them. This determination was entirely theirs because God hardened his heart, letting Sihon fully go the way of a sinful heart. Consequently the Lord delivered them all into Israel’s hand.