Englishman,
I will assume you meant if R’Chaim Kanievsky was 20 years old, because obviously soldiers need to be young and healthy. Not everyone can join hatzoloh but everyone can participate in mitzvos they do by donating money.
Akuperman,
I will even go so far as to admit of the possibility of an Arab state in E”Y that allows a Jewish minority to remain relatively free of harassment. But before that happens there will be weeks of bloodshed, and when the progroms start the Arabs won’t differentiate between Chilonim and Chareidim, they will kill, maim and rape all Jews.
Derech HaMelech,
We are not allowed to rely on nissim. We have to make hishtadlus to the utmost and rely on HKBH for the rest. If we just sat and learnt and did not have an army we should not be surprised if we were slaughtered. The Rambam you cite is clearly referring to exceptional individuals, people who are the exception not the norm.