Machshavot HaLev - Yeshivat Lev HaTorah

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בלה תובשחמ
other part of the tefillin is not available at all!
Rav Soloveitchik explains that this halacha reflects the themes of these two parts
of the tefillin. Tefillin shel yad reflects the active element of a person’s avodat
Hashem, as the arm represents action. Tefillin shel rosh corresponds to the belief
system, the emuna and bitachon that form the foundation of one’s avodat Hashem.
There must not be any interruption between these two parts of the tefillin because
one must be consistent in one’s avodat Hashem, committed both to the beliefs and
actions that are required.
Perhaps this can help us answer our question in a similar vein. One who puts the
tefillin shel rosh before the tefillin shel yad implies that he is only committed to act
upon that which his mind can comprehend. A Jew’s commitment to God’s word,
however, must be total and not dependent upon his own understanding. When
we said “na’aseh ve-nishma,” we indicated our total commitment to acting upon
God’s commands even before we are able to fully understand the importance of
any particular mitzva. It is because the soldiers possessed such a high level of
confidence in Hashem’s guidance and our mesora that they merited such a high
level of Divine protection.