citizens are called up to do the job
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by whom? what are they supposed to do? shoot down missiles with their rifles?
The Navy and Marine Corp are both Constitutional Ari, the Army was meant to have a small professional Officer Corps and then Militia to be called up and led by those Professionals as the need arose.
The President. Dig out a copy of the Constitution and read what it says about militias. Then dig out a copy of the Militia Act of 1792 and read how the same guys who wrote the 2nd Amendment intended the Constitutional directive be carried out.
what are they supposed to do?
Defend the country.
shoot down missiles with their rifles?
Of course not. If not for the unconstitutional NFA laws, there would be plenty of hardware in citizens' hands to deal with missiles et al.
Remember: the Constitutional take on militias was written by guys who had functional cannons decorating their front lawns, and hung out with other guys who owned battleships - and wanted YOU to have the freedom to do/own the same.