Comentario sobre Números 21:22

אעברה בארצך LET ME PASS THROUGH THY LAND — Although they had not been bidden to open the negotiations with them by an offer of peace yet they first sought peace of them (cf. Midrash Tanchuma, Chukat 22).

Although they were not commanded… Meaning that although they were not commanded to do so, they made this request since they only wished to make a temporary traversal [of the land]. It was not similar to the surrounding cities where the Torah forbade [them to make peace] (see Devarim 20:16-17). Re’m, Nachalas Yaakov, Gur Aryeh and Divrei Dovid all raise the difficulty that surely Sichon was king of the Emorites and thus included among the seven nations. Therefore, why did they offer a peace overture given that in reference to the seven nations the Torah writes, “You shall not allow any soul to live” (ibid.)? The answer is that even in the Land of Israel itself, the prohibition, “You shall not allow any soul to live” only applies once they had already begun the war. Thus when it is taught that Yehoshua sent three messages to the Land of Israel, the first of which was “whoever wishes to make peace, come and make peace” it was before they had begun the war.

עד אשר נעבור גבולך, “until we cross your borders. (on the walk past it).” They had not even asked to traverse Sichon’s territory. They were going to walk around close to the borders. The same was the case with walking around the territory of the land of Edom. This is why they could refer to having had no objections from the Edomites, or for that matter from the Moabites (Compare Deuteronomy 2,29)

Let me pass through your land. They did not say, “Please let me pass” as they did regarding the King of Edom, for they were informing him that they had to pass and were not asking. Moshe informed Sichon because if Sichon would agree, he would not wage war with him. Moshe had no desire to conquer land on the other side of the Jordan River, for the reason stated in Sifei Parshas Eikev: The Holy One became angry at Dovid HaMelech because he conquered Syria before Eretz Yisroel. Furthermore, Moshe had another hidden reason, as explained in Devarim 2:27, that this caused a great evil to another generation.