The 58th prohibition is that we are forbidden from fearing the enemy1 at time of war and retreating before them. Rather, it is an obligation to strengthen one's heart and stand strong in the lines of battle. A person who turns away and retreats violates this prohibition.
The source of this prohibition is G‑d's statement,2 "Do not retreat before them." This prohibition is repeated in the statement,3 "Do not fear them." The same idea — not to retreat or turn back during battle — is repeated many times, because it is a situation in which real truth4 can triumph.
The details of this mitzvah are explained in the eighth chapter of tractate Sotah.5
