Earlier quoted context omitted.
That would be contrary to the whole spirit of Federalism. It would be like every EU country having a separate department of internal affairs which reports directly to Brussels, except, perhaps, even more politically inflammatory. I'm not saying I necessarily disagree with you, just noting that there's a big wall in the path of implementing the concept.
The US is a single country with a single military.
Not... entirely accurate.
Many states possess their own military forces [0], and state National Guard units are dual-chained to both state and national military leadership [1] (in different scenarios).
[0] https://en.wikipedia.org/wiki/State_defense_force#Active_sta...
[1] https://en.wikipedia.org/wiki/United_States_National_Guard#N...