Deutéronome 23:10

When you go forth in hosts against your enemies, beware of every evil thing, of strange worship, the, exposure of the shame, and the shedding of innocent blood.

When the camp goes forth The camp that goes out in battle against the enemy, bringing the Ark with it, is known as the “lesser camp” (as opposed to the “greater camp”, where the Ark was surrounded by partitions). When they went out in battle, they were close to the environs of the Ark. Clearly, to be “ritually impure due to a nocturnal event” means to have had a seminal emission. The juxtaposition of this passage to the previous one, which mentioned the Balaam affair, is due to the Midianite War (since they had “harassed” us on account of “their princess” [see Numbers 25: 18]), because the Ark was with them in the campaign against Midian.

every evil thing To the spirit, and to the body, like a seminal emission.

WHEN THOU GOEST FORTH IN CAMP AGAINST THINE ENEMIES, THEN THOU SHALT KEEP THEE FROM EVERY EVIL — “because Satan indicts [people] in the hour of danger.” This is Rashi’s language.
The correct interpretation regarding this commandment appears to me that Scripture is warning of a time when sin is rampant. The well-known custom of forces going to war is that they eat all abominable things, rob and plunder, and are not ashamed even of lewdness and all vileness. The fairest of man by nature comes to be possessed of cruelty and fury when the army advances against the enemy. Therefore, Scripture warned, then thou shalt keep thee from every evil. And by way of the simple meaning of Scripture this is an admonition against doing anything forbidden. And in the Sifre it is stated: 254. “I might think that Scripture is speaking of the laws of defilements and purities and tithes. It therefore says ‘ervah’ [an unseemly thing, which refers to unchastity].. Whence do I know to include idolatry, lewdness, bloodshed, and blasphemy? Scripture therefore says, then thou shalt keep thee from every evil. Or perhaps the verse is speaking of defilements and purities, and tithes? It says ervah (an unseemly thing):. just as ervah uniquely represents a deed for which the Canaanites were driven from the Land and which removes the Divine Presence [from Israel], so all deeds for which the Canaanites were driven from the Land and which remove the Divine Presence [from Israel] are included in the admonition of this verse. When Scripture states davar [thing, but literally ‘word’ — then thou shalt keep thee from every evil ‘davar’] it includes also ‘evil talk.’” This also is included in the purport of the verse we have explained, that besides the [specific] admonitions which are stated concerning these stringent sins, he added yet a [special] prohibition to an army that we guard against any of these sins so that the Divine Presence withdraw not from the Israelites who are there, just as he said, For the Eternal thy G-d walketh in the midst of thy camp Thus he who commits any of the great sins while in the army, those about which it is written, they have set their detestable things in the house whereon My Name is called to defile it, [he causes the Divine Presence to withdraw from Israel]. Moreover, [we are to avoid these sins] in order that the enemy should not overpower us because of our committing the very deeds that cause them to be driven from before us, this being the sense of the expression, and to give up thine enemies before thee. The Sages [in the Sifre] added evil talk [as derived from this admonition] in order that contention should not increase among them and smite them with a very great plague, [even] more than the enemy [will inflict upon them].