To summarize
according to katan, ujm, somejew and the literal reading of vayoel moshe –
even if the whole medina would be fully haredi and the hafets hayim himself would be PM , it would be yehareig ve’al yaavor to join the fully haredi IDF because it still would classify as an army of shmad
and therefore, when faced with a binary choice of
either chv’sh the wholesale slaughter of all inhabitants of EY versus enlistment ,
they would choose the former
according to the majority of talmidei hahamim –
joining ,in that case, would constitute a mitsva of hatsalat nefashot
.