So... required by personal sense of duty? I can go with that. If someone chooses to be an intentional, willing victim, though, that is their choice, and not my prerogative to overrule or override them.
Well, perhaps. However, there are such laws as the militia act, around since nearly the beginning of the of the republic, which requires compulsory service of a specific people within the militia when called.
Defending oneself, one's neighbor, one's community, is not contrary to the libertarian political philosophy. Otherwise, the philosophy would be just plain ol' anarchy.