This chapter contains regulations for war in general (vv1-15) and for war within the confines of the land God had given them (vv16-20).

‘The passage is describing “holy war” (sometimes called “Yahweh war”), a conflict initiated by the Lord, empowered by Him, and resulting in His appropriation of its spoils. Such warfare was undertaken to destroy peoples who were irretrievably beyond redemption and who were likely to contaminate Israel’s faith with their idolatrous practices (see 1:30; 7:2, 16; 19:1).’ (Apologetics Study Bible)

Deut 20:1 When you go to war against your enemies and see horses and chariots and an army greater than yours, do not be afraid of them, because the LORD your God, who brought you up out of Egypt, will be with you.

In vv1-4, the main point is that the people do not need to be afraid, because the battle is the Lord’s.  This is underlined by the role of the priest before battle, v2ff.

Deut 20:2 When you are about to go into battle, the priest shall come forward and address the army.

Deut 20:3 He shall say: “Hear, O Israel, today you are going into battle against your enemies. Do not be faint-hearted or afraid; do not be terrified or give way to panic before them.

Deut 20:4 For the LORD your God is the one who goes with you to fight for you against your enemies to give you victory.”

Deut 20:5 The officers shall say to the army: “Has anyone built a new house and not dedicated it? Let him go home, or he may die in battle and someone else may dedicate it.

It is clear from vv5-8 that Israel did not have a standing army at this time.  The soldiers were ordinary citizens.  Exemptions were given to various groups whose domestic responsibilities (or whose fear, v8!) excused them from military service.

Deut 20:6 Has anyone planted a vineyard and not begun to enjoy it? Let him go home, or he may die in battle and someone else enjoy it.

Deut 20:7 Has anyone become pledged to a woman and not married her? Let him go home, or he may die in battle and someone else marry her.”

Deut 20:8 Then the officers shall add, “Is any man afraid or faint-hearted? Let him go home so that his brothers will not become disheartened too.”

Deut 20:9 When the officers have finished speaking to the army, they shall appoint commanders over it.

Deut 20:10 When you march up to attack a city, make its people an offer of peace.

The terms of engagement, vv10-15, are reasonably humane for the period.

Deut 20:11 If they accept and open their gates, all the people in it shall be subject to forced labour and shall work for you.

Deut 20:12 If they refuse to make peace and they engage you in battle, lay siege to that city.

Deut 20:13 When the LORD your God delivers it into your hand, put to the sword all the men in it.

Deut 20:14 As for the women, the children, the livestock and everything else in the city, you may take these as plunder for yourselves. And you may use the plunder the LORD your God gives you from your enemies.

In the ancient world, soldiers were not paid a wage.  Rather, they were allowed a portion of the plunder from their conquests.

Deut 20:15 This is how you are to treat all the cities that are at a distance from you and do not belong to the nations nearby.

Deut 20:16 However, in the cities of the nations the LORD your God is giving you as an inheritance, do not leave alive anything that breathes.

Deut 20:17 Completely destroy them—the Hittites, Amorites, Canaanites, Perizzites, Hivites and Jebusites—as the LORD your God has commanded you.

The practice of total destruction (rather than taking prisoners or spoils) was used only sparingly in the ancient world.  In the case of the Israelites, total destruction was called for in the case of Jericho, Josh 6:17–24, Hazor, Josh 11:10–11, Zephath, Judges 1:17, and the Amalekites 1 Samuel 15:3.

See Num 31 for an account of this policy in action (but from a different point in the narrative).

Deut 20:18 Otherwise, they will teach you to follow all the detestable things they do in worshipping their gods, and you will sin against the LORD your God.

Enns observes that the stated rationale for this extermination is the protection of the Israelites (from the ‘detestable’ practices of the Canaanites) rather than the punishment of the Canaanites for those practices.  But it is difficult to see why we should distinguish so sharply between these two motives, especially as elsewhere (as Enns himself notes) the extreme sin of the Canaanites is held out to be the reason for their extermination (Gen 15:16; Deut 9:4), with the tipping point being Canaanite child sacrifice (Deut 12:31).  (On the latter point, Enns pleads that the Canaanites ‘had hardly cornered the market on this act’ – see 2 Kings 3:27 – but the truth is that such a practice is viewed with abhorrence throughout the OT).

Deut 20:19 When you lay siege to a city for a long time, fighting against it to capture it, do not destroy its trees by putting an axe to them, because you can eat their fruit. Do not cut them down. Are the trees of the field people, that you should besiege them?

Environmental damage was to be kept to a mimimum.

Deut 20:20 However, you may cut down trees that you know are not fruit trees and use them to build siege works until the city at war with you falls.

‘The regulations for war in ch. 20 need to be used with great caution when principles are sought for the conduct of modern wars. The first requirement is to distinguish holy war from other kinds, even in Israel. Holy war is a concept which applies only, once and for all, to Israel’s occupation of its God-given land. Even Israel’s wars in general are special, because at that period in the history of God’s dealing with humanity his people was also a nation, a political unit. Now that that people is a church, which fights no wars as such, no nation has a mandate to suppose that God marches in its ranks in the wars that it fights—even where those wars may reasonably be thought just.’ (NBC)

Christopher Wright discusses how the principle of love for neighbour can be operative in the context of war.  He makes two points.  Firstly, love for neighbour does not mean a facile ‘niceness’ that is incompatible with punishment of idolatry or murder.  Similarly, the meting out of divine justice on the Canaanites is not incompatible with God’s larger purposes regarding the blessing of all nations.  Furthermore, this justice is not incompatible with the compassion and protection afforded to the foreigner.  Secondly, we have to reckon with the fallenness of human nature within which love for neighbour has to operate.  Sometimes it is necessary ‘to work for the humane within the humane, to mitigate the worst effects of human sin’ not only in the face of the realities slavery and divorce, for example, but also (as in this chapter) in the face of terminal wickedness.