Deuteronomy 20:1

When you go forth to battle against your enemies, and see horses and chariots, and peoples proud, overbearing, and stronger than you, fear them not; for all of them are accounted as a single horse and a single chariot before the Lord your God, whose Word will be your Helper; for He brought you free out of the land of Mizraim.

The passage when you go out in war is juxtaposed to the previous passage because, although “you must not show mercy” [19: 21] upon the guilty, you must show mercy, when you go out in battle, upon the betrothed, the homebuilder, the planter, and the fainthearted.

WHEN THOU GOEST FORTH TO BATTLE AGAINST THINE ENEMIES, AND SEEST HORSES, AND CHARIOTS etc. This is a new commandment which he now declared to them as they came into battles. The purport of the verse, For the Eternal your G-d is He that goeth with you, to fight for you against your enemies, to save you is to admonish them against becoming faint-hearted and that they should not fear their enemies. He states that they are not to rely in this matter on their own strength, thinking in their hearts: We are mighty men, and valiant men for the war, but instead they are to turn their hearts only to G-d and rely on His help, thinking that He delighteth not in the strength of the horse, and He taketh no pleasure in the legs of a man, for The Eternal taketh pleasure in them that fear Him, in those that wait for His mercy., Verse 11. He states to fight for you against your enemies meaning that He will make them fall before you by the sword. He states to save you meaning that they will be spared in battle and that not a man among them will be missing, for it would be possible that they vanquish their enemies and that many of them, too, would die, as is the way of battles. Therefore Joshua cried out when about thirty and six men, Verse 5. fell in [the battle of] Ai, for in His commandatory war not one hair of their heads should have fallen to the ground, for the battle is the Eternal’s., 17:47. Now the priest who serves G-d is to admonish [the warriors] to fear Him, and give them assurance [of His help]. The officers, however, speak in the customary way of the world, lest he die in the battle, for in the normal course of events even some people of the group of the victors die. He commanded that these three categories [of people] return because one’s heart is on his [new] house, vineyard, and wife and he will [be the first to] flee.

כי תצא למלחמה, outside the boundaries of the land of Israel. When a war is fought inside the boundaries of the Land of Israel, i.e. a defensive war, no one is allowed to be exempt from army service; even a bridegroom fresh from the wedding canopy is called up. (Sotah 44)