Ultra-Orthodox typically means strict adherence to the Holy Book. In this case the Old Testament / Torah.
So, since the old books are replete with the Jewish people fighting in wars I’ve often wondered how they come to this position. How do they justify it?
Many great Biblical figures including Abraham, Moses, Joshua, and David fought just fine, thank you. There was a Biblical exemption for newlywed men (Deut. 24:5)
The Mishna (a compilation of Jewish teachings dating, it is believed, far back into Biblical times and put down in writing shortly after the turn of the era), includes a teaching that everyone must help defend the community.
A millenium later, Maimonides (”the Rambam”) wrote about the Tribe of Levi as being exempted from defense, military service. He went on to describe a (quite idealized, imho) broader group of religious scholars as possibly also being exempt, quote 1 below. But Maimonides also extolled the fighting powress of Abraham, for instance, quote 2 below. Other authorities or commentators (some “Achronim” or religious scholars in the centuries following Maimonides) put forward the idea that everybody must help defend the nation (such as in quote 3 below).
All in all, I side with the view that everybody must help defend the land. Bible/Torah scholars should, imho, participate in training and then actually serve when needed (they can be granted student deferments on a reasonable basis, but definitely they should pull their weight in the common defense imho).
I say that opinion with the very highest and greatest respect for the many sincerely religious people studying in these seminaries, academies, colleges, etc.
The greatest respect!
Still, as the Mishna says,”in an obligatory war everyone must go to war, even a groom from his chamber and a bride from her canopy.” (Misha Motah 44b), Deuteronomy notwithstanding. I concur with this Mishnaic POV, but no matter as regards most of the religious scholars. They need to help defend the land imho, and most of them they are not exempted by Deut. 24:5.