Deuteronomy 2:24

2:24 Get up, make your way across Wadi Arnon. Look! I have already delivered over to you Sihon the Amorite, king of Heshbon, and his land. Go ahead! Take it! Engage him in war!

Deuteronomy 20:1

Laws Concerning War with Distant Enemies

20:1 When you go to war against your enemies and see chariotry and troops who outnumber you, do not be afraid of them, for the Lord your God, who brought you up out of the land of Egypt, is with you.

Deuteronomy 20:20

20:20 However, you may chop down any tree you know is not suitable for food, and you may use it to build siege works against the city that is making war with you until that city falls.