based on Siach Shaul p. 457-8 (an address from 1938)
Hashem commanded Moshe: "Take the retribution of Bnei Yisrael from the Midianites, and then you will pass away" (Bamidbar 31:2). Before Moshe was to die and leave the nation, he was to carry out revenge for the actions surrounding Ba’al Peor. What was Moshe’s strength that made his participation important?
The midrash (Midrash Tannaim to Devarim 33:28) says: "It is not like the badad (alone) that Bilam said – "They are a nation that dwells alone" (Bamidbar 23:9) and not like the badad that Yirmiya said – "Alone I sat" (Yirmiya 15:17), but like that which Moshe said – "Hashem will lead them alone" (Devarim 32:12).
Bilam came with a claim and explained the Israelite exclusionism as follows: "Among the nations, they will not be considered." The nations stay away from Bnei Yisrael. If the nations embrace them, they will lose their uniqueness. It is not like Yirmiya’s aloneness either. "Because of Your hand [against me], I sat alone" (Yirmiya 15:17). We do not want an aloneness that is rooted in sadness and depression.
It was necessary to show the Midianites that we also have the ability to use strength. The behavior that the nations display and we do not is not because we are incapable but because we do not wish to act that way. However, we have no choice but to respond with force to those who rise up against us with treachery. This is the idea that Moshe wanted to entrench within the mindset of the nation.
The same was true in regard to Amalek. They came upon us in combat, and we were commanded to "to erase the memory of Amalek" (Devarim 25:19). However, this was only when our ability to use power was intact. After the Temple was destroyed, we only mention Hashem’s promise that He will erase them (Shemot 17:14). When we do not have the ability to act ourselves, we leave it up to Hashem. We realize that in the final analysis, it is Hashem’s vengeance that needs to be taken. Rabbi Chanina ben Tradyon said before his death: "If I were burnt by myself, it would be hard for me; now that I was burnt along with a sefer Torah, He who will take issue with the disgrace of the Torah, will take issue with my disgrace" (Avoda Zara 18a).
In the haftara, we read: "Sacred is Israel to Hashem, the beginning of His produce; he who devours them will be blamed" (Yirmiya 2:3). Even tithes (which reishit – the beginning – hints at) that are defiled are still considered holy, and non-kohanim who eat them are guilty. We live with this realization. If Moshe warned "and among the nations you will not find tranquility" (Devarim 28:65), Yirmiya said: "Going toward tranquility for Israel" (31:1). Indeed we are a "nation that is a survivor of the sword" (ibid.). Every day the sword devours, and every day has its sacrifice and its torment. But as a nation, we are survivors. "From a distance Hashem was seen to me, yet an eternal love I have loved you" (Ibid. 2). It is from a distance – the distant past and the distant future, but we survive because of the strength we extract from these distant times.