Numbers 31:6

Them, and Pinchas. This teaches that Pinchas was equivalent to all of them. But why did Pinchas go, while Elozor did not? The Holy One, Blessed is He, said, “The one who began the mitzvah by executing Kozbi, the daughter of Tzur, should complete it.” Another interpretation: He sought to avenge Yoseif, his maternal forefather, as it is said, “the Midianites sold him. . .” How is it derived that the mother of Pinchas was [descended] from Yoseif? Since it is said, “of the daughters of Putiel (פּוּטִיאֵל).” Of the children of Yisro, who fattened (פִּיטֵם) calves for idol worship, and of the children of Yoseif, who scorned (פִּטְפֵּט) his evil inclination. Another interpretation: He was annointed for warfare.

With the sacred vessels. This refers to the Ark and the Golden Headplate. Because Bil’am accompanied them, and caused the Midianite kings to fly through sorcery, he himself flying with them, he [Pinchas] displayed the Golden Headplate to them, in which the Divine Name was engraved, and they fell. This is why it is said, “. . .on those they had slain,” concerning the Midianite kings — they fell from the air on the slain ones. Similarly, concerning Bil’am, it is written, “. . .upon those they had slain” in the book of Yehoshua.

In his hand. In his possession. Similarly, “he took his entire country מִיָדוֹ, from his possession.”

And Mosheh sent them, a thousand of each tribe to the war, them and Phinehas bar Elazar the priest unto the war, with the Uraia and Thummaia consecrated to inquire for them, and the Jubilee trumpets in his hand for assembling, encamping, and ordering forward the host of Israel.

AND MOSES SENT THEM, A THOUSAND OF EVERY TRIBE. He did not send all the members of the army there, although the Midianites were a large people, and their cities were fortified, and very large., 13:28. The reason for this is that those who had sinned with the Moabite women were many, and they were not fit to execute the vengeance of the Eternal; therefore they chose those men who were known amongst their tribes as righteous men. Now Moses did not command them what to do, but merely told them to execute the vengeance of the Eternal on Midian, and he thought that they would let none of them remain or escape, but [would execute the vengeance of the Eternal] like the vengeance [they were commanded] in connection with Amalek or the vengeance [they were commanded] in connection with the seven nations [of the land of Canaan]. And therefore when Moses saw that they had spared the women and children and the cattle, he was angry about [the fact that they had left alive] the women who had had sexual intercourse, because the officers of the army should have killed them as the very first thing, both as execution of the vengeance [which they were now commanded to do], and in performance of the general law of the Torah [that a living thing which causes a man to sin must be killed, as it is said], and ye shall slay the beast. And when he saw that the people wanted [to keep] the spoil, he remitted [and allowed them to keep] the female children and the spoil., Verse 20.
It is also possible to explain that Moses was [only] commanded to Harass the Midianites, and Avenge the children of Israel of the Midianites, [but was not commanded to destroy them all like Amalek and the seven nations]. Therefore he sent only a small contingent [twelve thousand men] to strike at the unwalled towns and fell every good tree, and stop all fountains of water, and mar every good piece of land with stones as spoilers usually do, and he did not give them any specific commands except that they should execute whatever vengeance they were able to do — and G-d, blessed be He, Who controls [the course of all] battles, gave the Midianites and their [five] kings and cities into their hands. Therefore Moses was only angry about [their sparing] the women who had had sexual intercourse, and [he was] not angry about anything else, as a form of vengeance. Hence we understand why he was not angry about their sparing the children …

וישלח אותם משה אלף אלף למטה לצבא, he sent one thousand men of each tribe.

אותם ואת פינחס, after the 12.000 men had all been assembled he sent Pinchas to accompany them.

לצבא וכלי הקודש וחצוצרות התרועה בידו, he sent him to the army, as head, i.e. as the priest especially appointed in every war involving invasion of foreign territory. The various holy vessels, including the trumpets would be at his disposal during that period.

וישלח אותם משה, “Moses sent them forth;” (the 12000 men making up the punitive expedition against the Midianites) You, the reader, will recall that the Lord had instructed Moses personally to head this expedition. (Verse 2 in our chapter, where He also added that after having carried out this mission Moses would die.) Why did Moses delegate others to do what he had been commanded to do himself? The reason must have been that seeing that he had found refuge there from persecution by the Egyptian secret service and had even became the son-in-law of a prominent priest in Midian, he did not think that G–d had meant for him to lead this campaign, just as he had not been ordered to strike the Nile which had sheltered him but had delegated that task to his brother Aaron at the time. He acted according to the well known proverb: “do not bite the hand that fed you.” Another interpretation suggests that the Midian of which Cosbi had been a princess was not the same Midian as that which is described in the Book of Exodus. [The Midian which Moses fled to bordered the desert of Sinai, whereas the Midian which is featured in connection with Bileam and Balak is situated in the Arabian peninsula bordering the red sea. Ed.] The reason why Moses chose Pinchas to be the leader of this expedition was that seeing he had commenced with turning away G–d’s wrath from the people, he should be given a chance to complete this good deed by punishing the seducers of his people. (Compare Bamidbar Rabbah 23,4)