Shoftim

The Torah delineates the process for Bnei Yisroel to follow before they went to war. When all the soldiers were together, the officers gave some of those assembled a few ways to get out of serving. They announced, “Who just build a new house & didn't live in it? Go back home. Who planted a vineyard & didn't redeem it ? You also go back. Whoever is engaged to be married, go back home. Finally, they asked מי האיש הירא ורך הלבב ילך וישב לביתו , who is the man who is afraid & weak of heart? Return to your home.

Rashi relates the opinion of רב יוסי הגלילי who says that the fear referred to is the fear of the sins that one has. The Torah lists all the other things, too, so that the ones who fear sin won't be embarrassed to return home. By listing all the different reasons together, no one knows why the other person is leaving.
The gemara says that  one who talks between the head tefillin & the arm tefillin while putting on tefillin has a sin on his hands & goes back from the ranks of the warriors.
Why is this mitzva specifically singled out as the cause to send home the warriors? What's the connection between tefillin & the soldiers?

The פסקי הלכות להרא"ש says that the strengthening of the mitzva of tefillin gives strength to the soldiers. The Torah states וטרף זרוע אף קדקוד , he rips the shoulder as well as the head. Rabbeinu bachaya says that the B’nei Gad weren't afraid to live surrounded by the other nations on the other side of the Jordan because they were armed with two weapons: physical strength & the merit of the mitzvos they did. Chazal expound וטרף זרוע as the merit of the arm tefilin;  אף קדקוד as the merit of the head tefillin.  Rashi there says that B’nei Gad were such fierce warriors that they cut off in one blow with their swords, the head & the arm together.
Says הר"י וורמסר , that's why one can't interrupt between the head & the arm tefillin. Since the whole greatness of the warriors was that they could cut of the head & arm together in one blow, which in the posuk is analogous to tefillin, so tefillin, too, have to be done in one shot.

We see this connection between soldiers & tefillin in our days too. We see countless pictures of soldiers on tanks putting on tefillin stories or how soldiers found religion on the battlefield. A story that exemplifies this, that just happened in the war in Gaza

A true and amazing story told by Ohad Shaked:
I received a phone call on Sunday from “A,” one of the Iron Dome commanders who was a student of mine about 6 years ago. I was glad he called. “Where can a person learn Torah in Ramat Gan?” he asked me, leaving me baffled since he had drifted far from Torah and mitzvot. "I'm going to be released in a couple of months, and I want to learn in a Yeshiva - I saw the Almighty with my own eyes!” Nothing more and nothing less. “What happened?” I asked him. “A rocket was fired from Gaza. The Iron Dome can detect where the rocket is going to fall within a 200 meter radius. This particular rocket was heading to the Azrieli Towers, or to the railroad tracks. Either way, hundreds could have paid with their lives!

We sent the first “dome” and it missed. Then the second as well as the third dome missed. This was a very rare occurrence. To date, only two other such cases had occurred. I was in shock! We had four seconds before the rocket would hit. We had already informed and dispatched emergency services, the police and the fire department to the location.”

Then the commander spoke more excitedly as I kept on listening. "Suddenly, without any preliminary data from the Iron Dome system, (which calculates the possible wind currents, etc.) a strong eastern wind blew – a wind for which we had no logical explanation, and cast the rocket right into the sea. We were all in shock!!! I stood up and started screaming, ‘There is a G-d’! ‘There is a G-d!’ ‘There is a G-d’!!! I saw this miracle with my own eyes. No one told me about it; it was not reported to me. I saw the Hand of Hashem fling the missile into the sea!

Of course, this was not reported for security reasons. However, witnessing the miracles with our own eyes shows there is Hashem. I ran to one of the religious soldiers and asked him to put on tefillin. I took it upon myself to keep Shabbat, and that was the very best Shabbat I have ever had.” This is what he told me. I was so excited that it even brought a tear to my eye. "Ashrecha” (praisworthy are you) I said to him, “that you merited to witness this incident and to understand that it’s from Hashem”

The first thing this soldier did when he found Hashem was to put on tefillin. Our tefilin connect us to Hashem. Let's take this lesson to strengthen ourselves in our own battles by strengthening the mitzva of tefillin - our connection to Hashem!