Whose idea was it to send messengers to Sihon in Deuteronomy 2:26?
This was a miniature and a quick rerun of what happened to Pharaoh. Sihon was like Pharaoh.
24 “Set out now and cross the Arnon Gorge. See, I have given into your hand Sihon the Amorite, king of Heshbon, and his country.
Right from the onset, the end was fixed and told by God to Moses. God saw the end from the beginning.
Begin to take possession of it and engage him in battle. 25This very day I will begin to put the terror and fear of you on all the nations under heaven. They will hear reports of you and will tremble and be in anguish because of you.”
Horizontally, physical events had to take place to connect the dots:
26 From the Desert of Kedemoth I sent messengers to Sihon king of Heshbon offering peace and saying,
There was no explicit mention that God told Moses to send messengers. In any case, Moses proceeded according to the plan of God because God was with him to guide him.
This was how horizontal events were to unfold in order to achieve the end goal.
27“Let us pass through your country. We will stay on the main road; we will not turn aside to the right or to the left. 28Sell us food to eat and water to drink for their price in silver. Only let us pass through on foot— 29as the descendants of Esau, who live in Seir, and the Moabites, who live in Ar, did for us—until we cross the Jordan into the land the Lord our God is giving us.”
What's the point of sending the above message?
30 But Sihon king of Heshbon refused to let us pass through. For the Lord your God had made his spirit stubborn and his heart obstinate in order to give him into your hands, as he has now done.
God hardened Pharoah's heart before; now, God hardened Sihon's heart.
31 The Lord said to me, “See, I have begun to deliver Sihon and his country over to you. Now begin to conquer and possess his land.”
See, what did I tell you, Moses? Sihon would not cooperate so that you would achieve the following:
32 When Sihon and all his army came out to meet us in battle at Jahaz, 33the Lord our God delivered him over to us and we struck him down, together with his sons and his whole army. 34At that time we took all his towns and completely destroyed them—men, women and children. We left no survivors.
Whose idea was it to send messengers to Sihon?
The Lord was with Moses when he sent the messengers.
One way or another, ultimately, it was the Lord's idea. Moses was working out the Lord's plan concerning Sihon who was a type of Pharoah.