Numbers 31:21

Israelite soldiers who touched dead bodies became ceremonially unclean (ritually impure). They had to go through the required purification rituals.

These rituals were routine, but they were important. They also had to purify their equipment (see 5:1–4; 19:1–22).

abide without the camp seven days: whosoever hath killed any person . . . purify both yourselves and your captives--Though the Israelites had taken the field in obedience to the command of God, they had become defiled by contact with the dead. A process of purification was to be undergone, as the law required (Lev 15:13; Num 19:9-12), and this purifying ceremony was extended to dress, houses, tents, to everything on which a dead body had lain, which had been touched by the blood-stained hands of the Israelitish warriors, or which had been the property of idolaters. This became a standing ordinance in all time coming (Lev 6:28; Lev 11:33; Lev 15:12).

And do ye abide without the camp seven days. We have elsewhere seen, [209] that, if any one had touched a dead body, he was accounted unclean. Moses, by now extending the ceremony of expiation to lawful homicide, intimates how carefully we ought to abstain from shedding human blood. It was required of the Israelites that they should strenuously advance through the midst of carnage; but, inasmuch as it is in a manner contrary to the order of nature that men should be killed by men, as if they were raging against their own bowels, God would have some vestiges of humanity preserved even in just punishments, so as to put a restraint upon all cruelty in the abstract. Nor is it without cause that Scripture, even in commending heroic bravery, uses this form of expression, that "they have polluted their hands with blood," who have slain any of their enemies, i.e., in order that we may abhor all acts of homicide, as being repugnant to the preservation of the human race. Although, therefore, the Israelites had slain the Midianites not only justly, but by God's command, still, lest they should accustom themselves to the indiscriminate shedding of blood, they are commanded to purify themselves on the third and the seventh day, before they returned to the camp, so that their pollution should not infect the people. The reason for purifying the booty was different, viz., because the uncleanness of their vessels indicated how detestable was this people, whose very utensils, until they were purified either by fire or water, defiled every one by the mere touch. Lest, however, the soldiers should refuse to obey, or should comply unwillingly, Eleazar reminds them that nothing more was required of them than the observance of an old injunction. Nor is it to be doubted but that Moses designedly resigned the office of teaching to his nephew, because the interpretation of the law was hereafter to be sought from the mouth of the priest

And Eleazar the priest said unto the men of war, which went to the battle,.... To all the soldiers, officers, and common men:

this is the ordinance of the law, which the Lord commanded Moses; concerning the purification of unclean persons and things, hereby confirming what Moses had said.

To this end Eleazar, whose duty it was as high priest to see that the laws of purification were properly observed, issued fuller instructions with reference to the purification of the different articles, in accordance with the law in ch. 19. למּלחמה הבּאים, those who came to the war, i.e., who went into the battle (see at Num 10:9). "The ordinance of the law:" as in Num 19:2. The metal (gold, silver, copper, tin, lead), all that usually comes into the fire, i.e., that will bear the fire, was to be drawn through the fire, that it might become clean, and was then to be sprinkled with water of purification (Num 19:9); but everything that would not bear the fire was to be drawn through water. - The washing of clothes on the seventh day was according to the rule laid down in Num 19:19.