A command to enlist?

I am not talking about this in terms of a commandment. Of course it must be done, even if there is no section in the Shulchan Arba that states so. It is a civic duty towards a country that is in danger. The words of the Rambam about helping Israel from a time of trouble are irrelevant to the discussion in my opinion, because this is not a mitzvah war like the other mitzvah wars, but a duty of the Jewish people and it exists in every country.