You are already a member of a militia in the US - it's called the state militia, (which in NOT the National Guard). And while it falls outside of formal military service, (Regular military, Reserve, or Guard), it does exist and you are a part of it from ages 17 to 55 or so. And in some states even women are subject to it equally. There are contingencies upon contingencies that already exist for this and have for a very long time.
This is a decent, and not super complicated overview of most of the military organizations and how they interact.(https://www.youtube.com/watch?v=nAsZz_f-DUA) The state militias part come towards the end.
I am a bit familiar with this as a medic who asked a dumb question, I was told we were subject to, (though it takes a really major disaster), to being "called up" by the Dept of Homeland Security to go and supply aid if needed and where needed. If I remember correctly some few were either called up or were close to being called and assigned during the last major hurricane in New Orleans. I'm old and retired now and I am no longer subject due to age.
So perhaps you should get that musket and start training...........