Today's reading is Deuteronomy 17-20 (read it in the KJV or NIV)
Today's passage covers rules for witnesses and law courts; details about the qualities of future kings of the Israelites; a list of detestable practices, mostly dealing with divination; qualities of a true prophet; more information on the cities of refuge; and instructions for going to war.
Unless you're discussing the Israeli Defence Forces (IDF), most modern people picture Jews as a relatively pacifistic people. Jews have been shunted from country to country for centuries, rarely allowed to serve in militaries, the persecuted far more often than persecutor. The archetypal image of a Jew for many people is a black-frocked scholar worrying about some tricky piece of Biblical text. Even today, when Jews are for the most part no longer mandated into ghettos and particular jobs, when we pictures Jews as doctors or bankers, there is a certain cognitive dissonance between the concepts of "Jew" and "warrior."
However, today's readings prove that the modern notion of a pacifistic Israelite was not always the case.
Deut. 20 is wholly devoted to warfare: who should address the soldiers, who should be sent home, how to approach hostile cities, and what to do once the conquests are complete.
The first thing we note is that it is not the king but the priest who gives the rallying pep-talk to the rank-and-file. Partly, this is because the king may not even exist yet. While we learn earlier in today's readings (Deut. 17:14-20) that there will be a king once the Israelites settle in the new land, God makes no promises as to when this will be. But more importantly, having the priest address the men reinforces the idea that the Israelites' war, like their peace, is under God's control. In fact, the priest's message furthers this point. He tells them, and I paraphrase: "Don't be afraid of the enemy, because God is with you, to fight for you against your enemies and to protect you." (Deut. 20:3-4) It is because of God, and not because of the Israelites' own strength, that they will win against their enemies. Therefore, it is perfectly appropriate that the priest addresses them.
After the priest speaks, it is the officers' turn. Their job seems to be choosing which men shall remain and fight, and which shall be sent home. (Deut. 20:5-9) The general census (Num. 26) notes down all adult males, making no exceptions. It may be that some men are not suitable for warfare, and that is addresses here.
There are two main types of men who are singled out to return to their homes and stay away from the fighting. The first are men who have built a house but not yet dedicated it, who have planted a vineyard but not yet eaten from it, or who have married a woman but not yet consummated the marriage. In each of these cases, the man is told to return home to finish his business, lest another do it for him. Reading between the lines, it seems these three conditions are meant to address the youth who have not yet had time to settle into their new lives. These are men with new houses, new crops, perhaps a new wife, who were called away young to go fight in the Israelites' army. They are being given a small mercy, allowed to grow up a little and maybe father a son, before they are sent out onto the field.
The second type of person who is not allowed to fight in the Israelites' army is the cowardly man. The text gives the reason: "let him go and return unto his house, lest his brethren's heart faint as well as his heart." (Deut. 20:8) In other words, fear is contagious. The high priest has already told the men not to be afraid, because God is on their side. If a man is afraid, it is because he does not trust God, and his attitudes may contaminate his faithful neighbours.
Finally, there is the war itself. We have established in earlier chapters, and here it is reiterated, that when the Israelites conquer the nations of Canaan, there are to leave no person alive. (Deut. 20:16-18) If they do, it is possible the Israelites may begin worshipping the Canaanites' gods, and this would be intolerable.
However, the Israelites are not expected to keep their wars in the relatively small corner of Canaan. God also gives them rules for conquering cities "which are very far off from thee." (Deut. 20:10-15) In these, foreign cities, the Israelites are expected to be more tolerant: they must first offer peace terms to the offending cities. If they agree, all is well: they become the tributaries of the Israelites and serve their conquerers. If, however, they choose to go to war, the conquering Israelites (as, of course, they will be successful in their conquests, with God behind them) must kill all the men, but are allowed to take the women, children, and spoils for themselves.
We are left, then, with the image of a strong, fearless conquering nation. With God behind them and the assurances of their priest, they are undaunted even against superior numbers. Their young and their cowardly have been sent home, so that only hardened, mature men stand in the army. After conquering the nations of Canaan and putting them all to the sword, they have a reputation for ferocity and ruthlessness, even when conquering far-away lands. In other words, you definitely wouldn't want to mess with these bankers.