John Gill 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)

And the Lord said to me
After or about the time when the messengers were sent to Sihon, perhaps when they had returned and had brought his answer:

behold, I have begun to give Sihon and his land before you ;
by hardening his heart, which was a sure token of his ruin, and a leading step to the delivery of him into the hands of Israel:

begin to possess, that you may inherit his land ;
move towards it and enter into it, not fearing any opposition made by him.