Deuteronomy 23:12

outside the camp: Human waste is a normal bodily function. However, in Israel’s law it made a person ritually unclean (not fit for worship for a time).

Because God is holy, the camp had to remain clean. For this reason, people had to go outside the camp when relieving themselves.

When the host goeth forth against thine enemies, then keep thee from every wicked thing--from the excesses incident to camp life, as well as from habits of personal neglect and impurity.

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. From moral pollution (Deu 23:9): When the host goes forth against thy enemy then look upon thyself as in a special manner engaged to keep thyself from every evil thing. 1. The soldiers themselves must take heed of sin, for sin takes off the edge of valour; guilt makes men cowards. Those that put their lives in their hands are concerned to make and keep their peace with God, and preserve a conscience void of offence; then may they look death in the face without terror. Soldiers, in executing their commission, must keep themselves from gratifying the lusts of malice, covetousness, or uncleanness, for these are wicked things - must keep themselves from the idols, or accursed things, they found in the camps they plundered. 2. Even those that tarried at home, the body of the people, and every particular person, must at that time especially keep from every wicked thing, lest by sin they provoke God to withdraw his presence from the host, and give victory to the enemy for the correcting of his own people. Times of war should be times of reformation, else how can we expect God should hear and answer our prayers for success? Psa 66:18. See Sa1 7:3.
II. From ceremonial pollution, which might befal a person when unconscious of it, for which he was bound to wash his flesh in water, and look upon himself as unclean until the evening, Lev 15:16. A soldier, notwithstanding the constant service and duty he had to do in the camp, must be so far from looking upon himself as discharged from the observance of this ceremony that more was required from him than at another time; had he been at his own house, he needed only to wash his flesh, but, being in the army, he must go abroad out of the camp, as one concerned to keep it pure and ashamed of his own impurity, and not return till after sunset, Deu 23:10, Deu 23:11. By this trouble and reproach, which even involuntary pollutions exposed men to, they were taught to keep up a very great dread of all fleshly lusts. It were well if military men would consider this.
III. From natural pollution; the camp of the Lord must have nothing offensive in it, Deu 23:12-14. It is strange that the divine law, or at least the solemn order and direction of Moses, should extend to a thing of this nature; but the design of it was to teach them, 1. Modesty and decorum; nature itself teaches them thus to distinguish themselves from beasts that know no shame. 2. Cleanliness, and, though not niceness, yet neatness, even in their camp. Filthiness is offensive to the senses God has endued us with, prejudicial to the health, a wrong to the comfort of human life, and an evidence of a careless slothful temper of mind. 3. Purity from the pollutions of sin; if there must be this care taken to preserve the body clean and sweet, much more should we be solicitous to keep the mind so. 4. A reverence of the divine majesty. This is the reason here given: For the Lord thy God walketh by his ark, the special token of his presence, in the midst of thy camp; with respect to that external symbol this external purity is required, which (though not insisted on in the letter when that reason ceases) teaches us to preserve inward purity of soul, in consideration of the eye of God, which is always upon us. By this expression of respect to the presence of God among them, they were taught both to fortify themselves against sin and to encourage themselves against their enemies with the consideration of that presence. 5. A regard one to another. The filthiness of one is noisome to many; this law of cleanliness therefore teaches us not to do that which will be justly offensive to our brethren and grieve them. It is a law against nuisances.

Deuteronomy 23:9-14

9. When the host goeth forth against thine enemies, then keep thee from every wicked thing.

9. Quum egressus fueris in exercitu contra hostes tuos, cave ab omni re mala.

10. If there be among you any man, that is not clean by reason uncleanness that chanceth him by night, then shall he go abroad out of the camp, he shall not come within the camp:

10. Si fuerit in te quispiam non of mundus casu nocturno, egredietur extra castra, nec ingredietur in medium castrorum

11. But it shall be, when evening cometh on, he shall wash himself with water: and when the sun is down, he shall come into the camp again.

11. quum autem aspexerit vesperum, lavabit se aqua: et quum occubuerit sol, ingredietur castra.

12. Thou shalt have a place also without the camp, whither thou shalt go forth abroad:

12. Locus etiam erit tibi extra castra, egredierisque illuc foras.

13. And thou shalt have a paddle upon thy weapon; and it shall be, when thou wilt ease thyself abroad, thou shalt dig therewith, and shalt turn back and cover that which cometh from thee:

13. Paxillus item erit tibi inter vasa tua, et quando desidebis extra, fodies ipso, et conversus operies excrementa tua.

14. For the LORD thy God walketh in the midst of thy camp, to deliver thee, and to give up thine enemies before thee; therefore shall thy camp be holy: that he see no unclean thing in thee, and turn away from thee.

14. Jehova enim Deus ambulat per medium castrorum tuorum, ut eripiat te, et tradat inimicos tuos coram te. Sit igitur in castris tuis sanctitas, ne videat in te turpitudinem aliquam, et avertatur abs te.

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. We know how greatly laws are disregarded during war, when all things are under the control of violence rather than reason; and we know that much license is wont to be given to soldiers, which would be by no means tolerated in peace. God would remedy this evil by requiring the Israelites to aim at the same purity in war as in peace; for this is a special law which forbids their being dissolute and unruly in war-time, as He has before condemned all impurity in general, as if He had said, that under no pretext would they be excusable, if they neglect the duty of cultivating habits of purity. For He does not command them to be cautious in the army and in the camp, as if they might sin with impunity when at home, but admonishes them that God would by no means excuse them although they should allege the necessity of war. Much more would the crime be aggravated, if they should pollute themselves in peace and when their minds were calm. Whence we gather that it is vain to catch at empty excuses for the violation of God's commands in any respect; for, however difficult the performance of duty may be, still God never resigns His rights. Now, if war, which seems to dispense with laws, does not excuse crime, much greater, as I have said, shall their guilt be accounted, who in a tranquil condition of life are licentiously carried away by sin.

10. If there be among you. He enumerates two kinds of pollution, whereby the Israelites may know what is meant by their keeping from the "wicked thing." First, He pronounces to be unclean, and casts out of the camp those who may have had a filthy dream, until they shall have washed themselves in the evening. Secondly, He forbids them to defile the camp with what passes from the bowels; and not only this, but, even when they have gone outside the camp, He commands them to bury their excrement beneath the earth, lest any filthiness should appear. Yet it is probable that, by synecdoche, everything is referred to which rendered men unclean and polluted. But Moses, speaking as to soldiers, considered it sufficient to tell them briefly, that although they might be occupied with war, cleanliness must still be attended to. By "what chanceth at night," all are agreed in understanding a flow of semen; from whence we infer how greatly impurity defiles a man, since uncleanness is contracted even from foul dreams. As to the second part, some desire to appear quick and clever by attacking Moses, because he has introduced among the precepts of holiness, that none should relieve his bowels in the camp. Forsooth, they say, the smell might offend the nostrils of God! But their silly petulance is easily rebutted; for God would by such rudiments keep His ancient people in the way of duty, lest liberty even in the most trifling things should lead them onwards to audacity. If they had been permitted to defile every part of the camp, the people would presently have been hardened against filthiness of every sort. Thus they were held back by this rein, that they might more earnestly apply their minds to spiritual integrity. They also are mistaken who suppose that this was a sanitary precaution, lest the smell should produce diseases, and be injurious to their bodily health. For Moses plainly declares that he not only had regard to what was wholesome, or even to what was decent in the eyes of men; but rather that he would accustom the people to abhor uncleanness, and to keep themselves pure and unpolluted -- for he adds, that God presided in the camp, to protect them from the power and assaults of their enemies; and that they should fear, lest, if they should contaminate the camp, He would be offended with their filthiness and forsake them. The sum is, that when they have need of God's assistance, and are engaged in war against their enemies, the pursuit of holiness must not be omitted or neglected even in the midst of arms.

Thou shalt have also a place without the camp,.... A place prepared, as the Targums of Onkelos and Jonathan, provided on purpose for the use hereafter suggested; so Ben Melech:

whither thou shalt go forth abroad; to do the necessities of nature, which they were to do without the camp, not in any place they thought fit and most convenient, but what was appointed for that purpose.

To wit, to ease thyself, as it follows, .

Preservation of the Purity of the Camp in Time of War. - The bodily appearance of the people was also to correspond to the sacredness of Israel as the congregation of the Lord, especially when they gathered in hosts around their God. "When thou marchest out as a camp against thine enemies, beware of every evil thing." What is meant by an "evil thing" is stated in Deu 23:10-13, viz., uncleanness, and uncleanliness of the body.
Deu 23:10-11
The person who had become unclean through a nightly occurrence, was to go out of the camp and remain there till he had cleansed himself in the evening. On the journey through the desert, none but those who were affected with uncleanness of a longer duration were to be removed from the camp (Num 5:2) but when they were encamped, this law was to apply to even lighter defilements.
Deu 23:12-13
The camp of war was also not to be defiled with the dirt of excrements. Outside the camp there was to be a space or place (יד, as in Num 2:17) for the necessities of nature, and among their implements they were to have a spade, with which they were to dig when they sat down, and then cover it up again. יתד, generally a plug, here a tool for sticking in, i.e., for digging into the ground.
Deu 23:14
For the camp was to be (to be kept) holy, because Jehovah walked in the midst of it, in order that He might not see "nakedness of a thing," i.e., anything to be ashamed of (see at Deu 24:1) in the people, "and turn away from thee." There was nothing shameful in the excrement itself; but the want of reverence, which the people would display through not removing it, would offend the Lord and drive Him out of the camp of Israel.