וַיֵּצֵא֩ סִיחֹ֨ן לִקְרָאתֵ֜נוּ ה֧וּא וְכָל־עַמּ֛וֹ לַמִּלְחָמָ֖ה יָֽהְצָה׃
Тогда Сихон выступил против нас, он и весь его народ, на битву при Иахазе.
Siftei Chakhamim
Places this section adjacent. . . It is surprising that Rashi brings this Midrash, as he only brings Midrashim explaining adjacent sections when the sections are written out of sequence. But here they need to be adjacent because both incidents occurred in the same place. (Re”m) Gur Aryeh explains that Rashi [brings the Midrash because he] is answering the question: Why did it not say, “Amalek went out,” as in (Devarim 2:32), “Sichon went out to war against us”? Therefore Rashi explains: “The Torah places this section adjacent to the previous verse. . . that the dog (Amaleik) will come and bite you. . ..” [In other words, it says] “Amaleik came” because they asked, “Is Hashem among us or not?” [Another explanation:] I wrote many times that Rashi only explains why verses are adjacent when there are extra words. Therefore Rashi comments here about the juxtaposition. Rashi is answering the question: Is “In Rephidim” not superfluous? It is written in v. 17:1, “They camped in Rephidim.” And it is written in 19:2, “They journeyed from Rephidim.” Consequently, whatever happened in between occurred in Rephidim. Perforce, “In Rephidim” is written here to tell us that the incident occurred in Rephidim [i.e., the verse of, “They tested Hashem, saying, ‘Is Hashem among us or not?’”] caused that “Amaleik came.” And that is why Rashi says, “Adjacent to the previous verse,” rather than, “Adjacent to the previous section.” Rashi is explaining the juxtaposition [of “Amaleik came”] specifically regarding this verse. (Nachalas Yaakov)
Rabbeinu Bahya
כי עז גבול בני עמון, ”for the border (territory) of the Children of Bney Ammon was powerful.” The verse is a simile. It is a reference to G’d’s instructions to the Israelites not to harass either the Bney Ammon or the Moabites (Deut. 2,19). This instruction made the territory of the Bney Ammon inviolate against any assault by the army of the Israelites. When the Torah mentions in verse 26 that Cheshbon was the capital of Sichon who had conquered it from the first king of Moav, this indicates that the reason Israel was allowed to conquer the lands of Sichon was that it was not included in the part of the territory concerning which G’d had warned the Israelites. This is also the meaning of the somewhat enigmatic statement in Chulin 60 עמון ומואב טהרו בסיחון, that “Ammon and Moav were purified through Sichon.” The conquest by Sichon of some of their territories invalidated the injunction to the Jewish people not to harass them. If the Torah saw fit to mention poetry by the local Emorites (verse 28) in which the victory over Moav is celebrated, our sages have reported to us details of the bravery and mighty deeds of Sichon and the power of the city Cheshbon by saying that even if Chesbon had been filled with only flies no ordinary army could have conquered it. They added that even if Sichon had been in the valley (without the added advantage of being on a mountain) no one could have overcome him (Tanchuma Chukat 23). They claim that even his name Sichon meant that his personal strength was tabletalk all over the country (the word סיחון sounding like שיחה). They deduce from the fact that the Torah makes special mention of the Israelites also killing Sichon’s son, [something which is included in the statement that no one escaped. Ed.] that this son was a mighty warrior in his own right already at that time. Possibly this is deduced from the fact that in Deut. 2,32 the Torah writes בנו instead of בניו as here when describing the fate of Sichon’s sons (son). According to Bamidbar Rabbah 19,29 although both Og and Sichon were Emorites they were so sure of themselves that they did not come to each other’s assistance in their fight against Israel. According to Tanchuma end of Chukat, Sichon’s son was even tougher than his father.