“…And you shall bind them for a sign upon your hand, and they shall be for a reminder between your eyes…” – Devarim 6:8

וּקְשַׁרְתָּם לְאוֹת עַל יָדֶךָ וְהָיוּ לְטֹטָפֹת בֵּין עֵינֶיךָ – דברים ו,ח

Our Sages taught that the mitzvah of Tefillin scares away the enemies of the Jewish people. The Talmud (Berachos 6a) declares, “It is written: ‘And all the peoples of the earth will see that the name of G-d is called upon you, and they will fear you (Devarim 28:10),’ …This refers to the Tefilin of the head.” Even in the event that our enemies attack us, G-d forbid, we are told that the merit of the mitzvah of Tefillin will give our soldiers remarkable strength in battle. “By fulfilling the mitzvah to wear the Tefilin [on the arm and on the head,] those going to war will see the fulfillment of [Moshe’s blessing to the warriors of Gad,] ‘May he tear off the arm [of his enemy] as well as his head’ (Devarim 33:20)” (Rabbeinu Asher, Laws of Tefillin 15).

Notably, in the first scenario the Talmud states that the enemies will be scared off by the Tefillin of the head alone. (The function of the Tefillin of the arm in this instance is that it facilitates the donning of the Tefillin of the head, [see Tzafnas Pa’aneiach, Hilchos Tefillin 4:4]). In the second instance, however, the Tefillin of the arm and of the head are individually effective, helping those at battle “tear off the arm [of the enemy] as well as his head,” respectively.

In light of the above, we can explain a noticeable difference between the commandment of Tefillin as it is stated in the Book of Shemos (13:9, 13:16), and how it is stated in the Book of Devarim (6:6, 11:18). In Shemos, the Torah describes the Tefilin of the arm and of the head as two components of one unit, “it shall be to you as a sign upon your hand and as a remembrance between your eyes.” In Devarim, however, the Torah states, “you shall bind them as a sign upon your hand, and they shall be for a reminder between your eyes”—distinguishing them as two distinct acts and obligations.

In Shemos, when the conquest of the Land of Israel was intended to be led by Moshe, both Tefillin together would have had a singular effect: the nations would have been afraid to go to war against Bnei Yisrael. As Bnei Yisrael sang at the Sea, “All the inhabitants of Canaan melted…” (Shemos 15:15). In Devarim, however, Moshe knew that the conquest of the Land would be (led by Yehoshua) and would require going to battle. Accordingly, he ascribed to the Tefillin two distinct mitzvos and merits.

—Likutei Sichos, vol. 9, pp. 55-56