Even the Dor Hamidbar and David Hamelech needed armies with them. We believe that T’fillos and Torah are the Ikar, but even our highest-level generations had armies as well.
And Avraham threw sand at the four kings. Sancherev’s army was wiped out by Hashem directly.
The problem is not that we have an army, but that those who learn Torah for the protection of other Yidden don’t learn with enough Hasmadah to merit that sort of protection. The reason why we need an army is not because of the Arabs, but due to our own Chesronos. Just look at everone taking Bain HaZmanim! Bain Hazmanim is not to vacation, but to learn a different Limud then your normal Sedarim (famous quote from Rav Gifter), but to keep the Hasmada up. Time is wasting, and there is much work to be done!
As per the hypothetical terrorist, if Yidden would learn with enough Hasmadah, the Arab with the gun would never make it to the Beis Medrash. Hashem is perfectly capable of protecting us without an army. Furthermeore, no one would look up for their sefer when he started shooting (like it was in Kelm), but the bullets would never make it (if we deserved that level of protection).