There are two possibilities (as regards civil conflict).
If Trump is impeached and removed from office, presuming that there are 218 legitimate votes in the House to send articles to the Senate, and 67 validly electing sitting US Senators vote to convict, then, Mike Pence is sworn in, Mattis remains as SECDEF, the Joint Chiefs all support Pence and Mattis, and therefore the chain of command remains intact.
Under those circumstances, I believe that commissioned officers will follow orders transmitted through the chain of command and their troops will follow their officers.
And if armed civilians rise in rebellion, I believe they will be slaughtered. Easily.
The second possibility is a non-legal or extra-legal removal of the President, with him surviving (a coup). This is a distinct possibility. The Acting President (I presume Pence) would appoint a new SECDEF, and then you would have the situation which existed in April 1861, with commanders choosing sides and armies in the field.
In that case, all bets are off.
“Look, I’ve been looking at this issue for years.”
You haven’t looked hard enough.
Answer the question: What makes you think Marines are going to march out and kill their families? Do you think Marines are the second coming of the Nazi gestapo?
Mathematically impossible.
Do you know how many tanks and attack aircraft are operated by all the armed services? Take that number and divide it into 320 million and get back to me. It would take 10 divisions plus support to clamp down on ONE state in rebellion let alone 50!
You guys keep thinking secession and open rebellion, the plan would be to peserve Congress the Presidency and the supreme Court. Convene in hiding and vote various blue counties out of the union.