Or HaChaim on Deuteronomy 20:8/אור החיים על דברים כ׳:ח׳

1 מי האיש הירא, "Who is the man who is afraid, etc." We are told in Sotah 44 that our verse speaks of soldiers who are afraid to die in battle due to sins they had committed. We find such a concept in Isaiah 33,14: "Sinners in Zion are frightened." It is natural that people who have sinned should be worried. Even people who are not aware that they did commit sins would start to worry about such a possibility when they go to war, on a dangerous mission. Shulchan Aruch Or Hachayim 54 mentions that one has reason to worry even if one only knows that one spoke during certain parts of the prayers when one should not have interrupted one's prayer by talking. The reason is simple. When one goes into battle one needs a miracle in order not to be hurt. If one is guilty of sins which one has not atoned for one cannot expect G'd to perform a miracle.