Why does the Torah ascribe the message here to Moshe, and in Bamidbar 21:21, to Yisrael?
Rashi: Refer to Bamidbar 21:21:1:1.
Rosh citing a Midrash: Moshe sent a message of Shalom, and Yisrael sent a message of war. R. Levi (or Reish Lakish) said, there was no argument. In one letter they wrote Shalom at the top, and war at the bottom (i.e. if you do not consent) "Ani Shalom... Hemah la'Milchamah" (Tehilim 120:7).
What prompted Moshe to send a message of peace to Sichon? What is "Midbar Kedeimos" referring to?
Rashi #1: He took his cue from (the episode with) the Torah. 1 When Hakadosh-Baruch-Hu came down to Har Sinai to give the Torah to Yisrael, He first offered it to Eisav and Yishmael - even though He knew that they would not accept it. Hence, Moshe sent a message of peace even though Hashem did not command him to. 2
Rashi #2: He took his cue from Hashem who preceded the world, who could well have sent a flash of lightning to destroy the Egyptians, but who sent Moshe from the desert to calmly warn Pharaoh to send His people out.
Da'as Zekenim, Hadar Zekenim, Rosh: Midbar Kedeimos is a place that was a Midbar Kodem (before) Hashem brought water from the rock, and now it is a source of water.
How could Moshe send to Sichon a message of peace after Hashem had commanded him to go and fight?
Moshav Zekenim (29): Moshe did so [to avoid] Chilul Hashem.
Devarim Rabah (Shoftim 5)