Chareidim in the IDF – Now that there's Frum Battalions is it an obligation to join? – Hear from the inside!!!

Now that there’s a Frum brigade what’s the justification of Frum people at least who aren’t learning not to go to the army?
If there’s a shortage of soliders why wouldn’t yeshiva bochurim be obligated to join – isn’t Pikuach Nefesh Doche the whole Torah?
If the Frum will become a majority in the country what will you do then?
If Torah obligates hishtadlus, what is the minimum practical hishtadlus a nation must take to prevent harm? Can tefillah alone replace defensive capability?
If the IDF ceased to exist tomorrow, what do you realistically foresee happening to Jewish cities like Bnei Brak, Ashdod, or Yerushalayim?