Sefer HaChinuch ספר החינוך

To not be terrified or to be afraid in war: That we have been prevented to not be terrified and be afraid of the enemies at the time of war and not to run away from them. Rather the obligation upon us is to strengthen ourselves against them and to stand in front of them. And about this is it stated (Deuteronomy 7:21), “You shall not be terrified in front of them.” And the prevention was repeated in another place, in its stating (Deuteronomy 3:22), “You shall not dread them.”

It is from the roots of the commandment that everyone in Israel should place his trust in God, may He be blessed, and not be afraid for his body in a situation that he can give glory to God, blessed be He, and to His people.

The laws of the commandment — for example, that which they, may their memory be blessed, said (Mishneh Torah, Laws of Kings and Wars 7:15) that a man not think at the time of war about his wife, nor his children nor his money, but rather he clear his [mind] from everything, to [focus on] the war. And he should further think that all the blood of Israel is dependent upon him, and [so] if he is afraid and “he pulls back his right [hand],” it is as if he spilled the blood of all of them — and like the matter that is written (Deuteronomy 20:8), “and that the heart of his brothers not melt like his heart.” And it is explicit in the words of the tradition, “Cursed be he who makes the Lord’s work a fraud; cursed be he who withholds his sword from blood” (Jeremiah 48:10). And [also] that which they, may their memory be blessed, said (Mishneh Torah, Laws of Kings and Wars 7:15) that anyone who fights with all of his heart and intention to sanctify God is assured that he will not find injury; and it will be a merit for him and his children, that his house will be established in Israel, and he will merit life in the world to come. And [it is] like the matter that is written (I Samuel 25:28), “for the Lord will surely make a faithful house for my master, since my master fights the wars of the Lord, etc.” And the rest of the details of the commandment are in the eighth chapter of Sotah. (See Mishneh Torah, Laws of Kings and Wars 6.)

And this commandment is practiced by males — as it is upon them to fight — at the time that Israel are upon their Land. And one who transgresses this and begins to think and to ruminate and to bewilder himself in war has violated this negative commandment (see Ramban’s gloss on the Sefer HaMitzvot, Negative Commandments 38) and his punishment is very great, as we have written.