What is the UCMJ's take on prior service and Retiree's joining Militia's ?

Responding to topics such as these concern me somewhat, as everything posted "online" has the potential to become part of public record, and therefore used against one at a later date. For that reason, let me qualify all to follow as merely academic and hypothetical.

First, what statute of law or regulation as per UCMJ would we be concerned with here? Second, what is a "militia", and how does it differ from say, a "Gun Club" or the Boy Scouts of America? Finally, if said hypothetical militia formed itself for the purpose of presenting armed opposition to any official arm of local, state, or federal law enforcement...let alone the Military, isn't the question of its "legality" somewhat moot?

Georgetown Law provides the following:

https://www.law.georgetown.edu/icap/wp-content/uploads/sites/32/2018/04/Prohibiting-Private-Armies-at-Public-Rallies.pdf

Clearly, under various forms of state law, there are existing statutes to be concerned with. The key issues here appear to be regarding subordination to legally established authorities, the intent of any acts perpetuated by any said "militia", and the various laws governing the use of deadly force.

A group of people paying for "tactical" training, or assembling regularly for that purpose are not (depending on the laws of their state) ostensibly doing so as a "militia". Although I am not an attorney, I imagine attempting to prosecute any of the same as such, without evidence of breaking the aforementioned laws, would be problematic at best.

Finally, if (once again, hypothetically speaking) any such group were to assemble and act while armed, towards opposing established authority...it seems logical to me that at that point, one is "illegally at war" with those authorities; neither protected by the Geneva Convention, nor relieved of obligations as citizens of the government backing the same authorities. That is very arguably different from what occurred during our Civil War, where individual state legislatures left the "Union", and then established a separate "nation"...which unless I'm mistaken, was never officially "recognized" (much like the Continental Congress at the outbreak of the Revolution). In that case, I believe it would ultimately come down to which "side" won said conflict...as to whether or not those actions would later be judged "illegal", or "patriotic".

Absolutely nothing! So long isn’t one of those extremist one.

As long as they do not advocate the violent overthrow of the government.