Deuteronomy 20:2

The priest] It is implied that the priests accompany the hosts of Israel into battle: see on Numbers 10:9. Hence the Heb. phrase 'to consecrate a war, or warriors,' usually rendered to 'prepare': see Joel 3:9 mg.; Isaiah 13:3.

5-9. From Numbers 1:3 it would appear that all able-bodied men from twenty years of age were liable to military service. But the evils of compulsory service were obviated by the rule laid down in this passage exempting certain classes. There was (1) the man who had built a new house or planted a vineyard, and had as yet got no return for his outlay. The law exempting him for a time was an encouragement to those who by personal outlay increased the material resources of the country. (2) A man who was betrothed or newly married was exempted for a year (cp. Deuteronomy 24:5). (3) The fearful and fainthearted were discharged. Fear is infectious, and the presence of such persons in the host would be a source of weakness and danger. For an instance of the observance of this rule see Judges 7:3. It is implied that a sense of honour will protect this law from being abused.