וְאָמַ֤ר אֲלֵהֶם֙ שְׁמַ֣ע יִשְׂרָאֵ֔ל אַתֶּ֨ם קְרֵבִ֥ים הַיּ֛וֹם לַמִּלְחָמָ֖ה עַל־אֹיְבֵיכֶ֑ם אַל־יֵרַ֣ךְ לְבַבְכֶ֗ם אַל־תִּֽירְא֧וּ וְאַֽל־תַּחְפְּז֛וּ וְאַל־תַּֽעַרְצ֖וּ מִפְּנֵיהֶֽם׃
I powie im: "Słuchaj Israelu! występujecie teraz do walki z wrogami waszymi; niechaj nie zmięknie serce wasze, nie obawiajcie się, i nie trwóżcie się, i nie drżyjcie przed nimi!
Mishneh Torah, Negative Mitzvot
That those engaged in warfare shall not fear their enemies nor be panic-stricken by them during the battle, as it is said, “….do not tremble, nor be ye terrified because of them” (Deut. 3:22; 7:21; 20:3).
Sefer HaChinukh
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.”
Sefer HaChinukh
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.