8. The tefillin are placed on the left hand, the weak hand. This is to remind us that man is weak and yetzias Mitzrayim wasn't due to the hand of man. It was solely the hand of Hakadosh Baruch Hu, with His wondrous miracles, as it states (Shemos 15:6) אויב תרעץ 'ה ימינך בכח נאדרי 'ה ימינך, "Your right hand, Hashem, is most powerful. Your right hand, Hashem, crushes the enemy." Bnei Yisrael were compared to a weak hand, יד כהה, and they didn't do anything at all.
We don the tefillin shel yad before the tefillin shel rosh. The Shaul u'Meishiv zt'l (Divrei Shaul, Matos) explains that the tefillin she rosh represents Hashem’s special hashgachah pratis over the Jewish nation. This is implied in the pasuk (Devarim 28:10) ממך ויראו עליך נקרא 'ה שם כי הארץ עמי כל וראו, "All the people of the world will see that the name of Hashem is called upon you and they will fear you," and the Gemara (Brachos 6.) says that this refers to the tefillin on the head. The tefillin shel Rosh proclaims to the world and to us that Hashem watches over us with hashgachah pratis. How do we merit this hashgachah pratis? Through learning the lesson of the tefillin shel yad on the left hand. We believe we are powerless and cannot do anything alone. Due to our firm belief and trust in Hashem, Hashem watches over us with hashgachah pratis. This is the lesson of the tefillin shel rosh, which follows the lesson of the tefillin shel yad.
With this information, the Shaul u'Meisiv explains the Chazal (Shir HaShirim Rabba 4:3) that says that if someone accidentally put on the tefillin she rosh before tefillin shel yad, he didn't go out to war against Midyan. The explanation is that they needed Hashem's hashgachah pratis when they went to war. They received this hashgachah pratis by putting on the tefillin shel yad first and remembering they are weak and can't do anything on their own. This is how they merited receiving Hashem's hashgachah pratis and protection at war. But if one accidentally mixed up the order and put on the tefillin shel rosh before the tefillin shel yad, he shouldn't go out to battle. Without a strong awareness of his weakness and total reliance on Hashem, he wouldn't merit the hashgachah pratis, represented by the tefillin shel rosh.
This concept is alluded to in the pasuk (Tehillim 94:18) יסעדני 'ה חסדך רגלי מטה אמרתי אם, which means that if a person says that רגלי מטה, I am weak, and I can't do anything on my own, he merits יסעדני 'ה חסדך, Hashem's hashgachah pratis. But when a person trusts in his strength, he will stumble.
