"When you draw near to the battle, the Kohen, shall come near and speak to the people. 'Hear O Israel, today you are approaching the battle against your enemies. Do not be faint-hearted, you shall not fear, you shall not be alarmed, and you shall not be terrified because of them. For HaShem, your G-d, is the One, Who goes with you, to fight for you against your enemies, to save you.'" The phrase "Hear O Israel" in this verse alludes to the recitation of "Shema Yisrael HaShem Elokenu HaShem Echad." The Torah advises the Jewish army to recite the "Shema," the declaration of the Divine Unity, when they go into battle. Reciting the "Shema" awakens the merit of Klal Yisrael, whom accept upon themselves the Divine Unity every day. As a result, Klal Yisrael will be saved from many troubles and calamities. HaShem conveys to them, 'Even if the recitation of the "Shema," morning and evening, is the only Mitzvah that you fulfill, it will suffice to protect you from your enemies.'
The Torah shields and saves Klal Yisrael. The recitation of "Shema" is always on our lips. Therefore it suffices as the merit that shields us and saves us in times of trouble.
(1) Reflect on the truth that the "Shema Yisrael" prayer is the most powerful weapon in the arsenal of Klal Yisrael. (2) When you engage in "life's battles," recite: "Shemah Yisrael HaShem Elokenu Hashem Echad." (3) Be encouraged by the awareness that the declaration of the Divine Unity will merit you -- and all Klal Yisrael -- with HaShem's protection and deliverance.
(Based on the commentary of the Chofetz Chaim and Rav Nachum M'horadnah)