When you are encamped against your enemies, then you shall keep yourself from every wicked thing.
9. When the host goeth forth. What he had taught with respect to the preservation of purity at home, and in time of peace, he now extends to times of war also, so that they might keep themselves clean from all defilement even in the midst of the clang of arms.
Israel was now encamped, and this vast army was just entering upon action, which was likely to keep them together for a long time, and therefore it was fit to give them particular directions for the good ordering of their camp. And the charge is in one word to be clean. They must take care to keep their camp pure from moral, ceremonial, and natural pollution. I.
Commenting on Deuteronomy 23:9-14
When the host goeth forth against thine enemies,.... An army of soldiers march in order to meet the enemy and fight him: then keep thee from every wicked thing; the Targum of Jonathan adds, by way of explanation,"from strange worship, uncovering of nakedness, and from shedding innocent blood;''that is, from idolatry, uncleanness of every sort, and murder; and all other wickednesses ought to be abstained...