Inner War Before Battle

בפסוק הירא ורך הלבב דרשו חז"ל מעבירות שבידו

On the verse "the man who is fearful and fainthearted" (Devarim 20:8), Chazal expounded that this refers to one who is afraid because of the aveiros in his hand.

The Sfas Emes cites the teaching of Chazal that the soldier sent home out of fear is really one who fears because of his own sins, not the natural fear of battle.

כי לא הי' שום פחד לבנ"י במלחמה אם לא הי' עבירה בידם

For Bnei Yisrael had no fear whatsoever in war unless there was an aveirah in their hand.

Bnei Yisrael had no reason to be afraid in war; fear could only arise as a result of an aveirah weighing on a person.

כי כל אנשי מלחמה היו קדושים וכשהי' הירהור חטא בא' מהם כתיב ולא ימס כו' לבב אחיו פי' שהי' מביא הירהור גם בלבם

For all the men of war were holy, and when there was a stirring of sin in any one of them, it is written "and let him not melt the heart of his brothers like his heart" (Devarim 20:8) — meaning that he would bring a sinful stirring into their hearts as well.

Since the soldiers were on a holy level, even a single person's sinful thought could spread, and the verse warns of this because his weakness would infect the hearts of those around him.

ורך הלבב הוא כל שלא ניתקן ב' היצרים כמ"ש בכל לבבך ואם לא נתהפך ב' הלבבות נאמר ילך וישוב לביתו פי' שעדיין יש לו מלחמה בנפשו כנ"ל:

And "fainthearted" refers to anyone who has not yet set right the two yetzarim, as it is written "with all your heart" (Devarim 6:5, meaning with both inclinations); and if the two hearts have not been transformed, it is said "let him go and return to his house" (Devarim 20:8) — meaning that he still has a war within his nefesh, as explained above.

Being fainthearted means a person has not yet harnessed both his yetzer hatov and yetzer hara to serve Hashem with all his heart; until both are transformed, the inner war remains and such a person is sent home.

Summary: The Sfas Emes explains, following Chazal, that the soldier described as "fearful and fainthearted" is sent home not because of ordinary battlefield fear but because of the aveiros he carries. Bnei Yisrael, when whole, had nothing to fear in war; fear only takes hold where sin has taken root. Because the men of war stood on a holy level, even one man's sinful stirring could spread to the hearts of his fellow soldiers, which is why the Torah removes him. Ultimately, faintheartedness reflects a person who has not yet transformed both his inclinations to serve Hashem with all his heart, leaving an unresolved war within his nefesh that disqualifies him from the war without.