Where do we find that Tefillin are the strength of Israel? The verse states, “All the nations of the land will see that Hashem’s name is upon you and they will fear you (Devarim 28:10).” … When? When they see the Tefillin that is on our heads. (Gemara Chullin 89a)
The forehead-Tefillin are also termed “our glory” (Gemara Berachot 11a) — they are the crown.
It is through keeping the Mitzvah of Tefillin … that Hashem grants the Jewish armies the blessing of Moshe Rabbenu… (Rosh, Hilchot Tefillin sec. 15)