John Calvin Commentary Deuteronomy 23:9

"When thou goest forth in camp against thine enemies, then thou shalt keep thee from every evil thing." — Deuteronomy 23:9 (ASV)

When the host goes forth. What He had taught regarding the preservation of purity at home and in time of peace, He now extends to times of war also, so that they should 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 usually given to soldiers, which would by no means be tolerated in peace.

God sought to remedy this evil by requiring the Israelites to aim at the same purity in war as in peace. For this is a special law that forbids their being dissolute and unruly in wartime, just as He has previously 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 were permitted to sin with impunity when at home; instead, He admonishes them that God would by no means excuse them, even if they allege the necessity of war. The crime would be all the more aggravated if they were to pollute themselves in peace and when their minds were calm.

From this we gather that it is vain to resort to empty excuses for the violation of God’s commands in any way; for, however difficult the performance of duty may be, God still never resigns His rights. Now, if war, which seems to override laws, does not excuse crime, much greater, as I have said, will their guilt be considered for those who in a tranquil condition of life are licentiously carried away by sin.