Or HaChaim on Deuteronomy 23:10/אור החיים על דברים כ״ג:י׳

1 כי תצא מחנה על איביך, "When a camp goes out against your enemies, etc." The Torah means that although there are nuances of transgressions which G'd does not go out His way to punish you for, and even if He does decide to punish you the punishment is very mild, the Torah informs us that at a time when there is general danger, such as when one goes out to war, one needs to be careful not to become guilty of violating even the smallest detail of G'd's commandments, i.e. מכל דבר רע, "from anything which is evil." The word דבר includes even causes which bring about evil. The Torah follows this up with the example of a soldier who became ritually unclean due to an involuntary nocturnal seminal emission. Our sages in Ketuvot 46 explain that sexual fantasies one has during the day often are the cause of such emissions during one's sleep. The reason the Torah wrote these two veרses next to one another is to warn us not to entertain such fantasies during the day so that we would not become defiled during the night.