Deuteronomy 2:30 (ESV)

30 But Sihon the king of Heshbon would not let us pass by him, for the LORD your God hardened his spirit and made his heart obstinate, that he might give him into your hand, as he is this day.

Sihon was not willing to let Israel pass through. For his own part it may have been because of fear, or because of confidence in his own military prowess. But Moses here, reflecting on the event, sees it in the context of the plan of God, who hardened Sihon's spirit. And no human power can stand against the Lord and his plan to save his people.