Charles Ellicott Commentary Deuteronomy 2:31

"And Jehovah said unto me, Behold, I have begun to deliver up Sihon and his land before thee: begin to possess, that thou mayest inherit his land." — Deuteronomy 2:31 (ASV)

Behold, I have begun to give Sihon. — Notice that in all the conquests of Israel Jehovah gave the order to begin the attack. (See Deuteronomy 7:2, and Note on Joshua 13:1.)