By both Amon and Mo'av, Hashem commanded Moshe not to attack them and relied on him to pass on the command to Yisrael. Why, by Edom, did he commmand Moshe to inform Yisrael?
Oznayim la'Torah: 'Midah k'Neged Midah'. Because, not only did Eisav desist from harming Ya'akov when they met after Ya'kov fled from Lavan, but he also orfered his four hundred men too to desist.
Why did Hashem refer to Eisav as Yisrael's brother?
Ramban: Because the lineage of Yisrael stems from Avraham, all of whose offspring are called 'brothers', since they were all circumcised. 1 That is why the Torah says "Do not hate an Edomi, since he is your brother" (23:8).
Oznayim la'Torah: Because he referred to Ya'akov as "Achi". 2
Ramban: With the exception of the sons of Avraham's concubines, Yishmael and B'nei Keturah, whom Hashem precluded when He said to Avraham (in Bereishis 21:12) "Ki be'Yitzchak Yikarei l'cha Zara". (This is difficult, since Nedarim 31a extrapollates "be'Yitzchak", 'and not all of Yitzchak', to exclude Eisav as well! See Rav Chavel's footnotes).
Refer to 2:3:1511:1.
Why does the Torah insert the words "ha'Yoshvim be'Se'ir"?
Rashbam: Because it was only the B'nei Eisav who lived in Se'ir who offered to sell them food and water. Those who lived in Edom responded to their request by coming out to meet them with the sword. 1
Rashbam: See Bamidbar 20, 21.
What is "Venishmartem Me'od" referring to?
Rashi: It is referring to waging war against Edom, as the following Pasuk explains.
Oznayim la'Torah: When an army sees that the enemy is afraid of them, they will take the opportunity to attack them. That is why the Torah writes here that, when they see that the B'nei Eisav are afraid of them, they should take great care to control the impulse to ttack them.
