How will a Civil War start? The Military Coup Scenario

This is how I  originally envisioned how a civil war would start.

The President would issue an order so unhinged and dangerous that a cadre of senior military officers, Constitutionalists, would remove the president and likely most of the cabinet from office (this would once have been Defence Secretary James Mattis).  A heavily-armed military could brush aside (not with total ease) the Secret Service (who are duty bound) and other regimists, at least initially.   I cannot see a neutral (let alone unifying) figurehead respected by all sides (no such person exists?) that they can find to assume the office - even as a short-term placeholder.  Certainly none of those in the line of succession will be: Vice-president likely removed with the President, Speaker of the House of Representatives seen as too partisan, as will the Senate President and all cabinet officers.  The coup leaders will most likely have to take over the role, even if their aim is to restore democracy to a extremely-polarised and divided nation.

If other units and officers in the military as well as police departments back the coup  leaders and don't act to counter this coup in essence, martial law will be imposed to keep order and the military government will have to be in command longer than they probably would want to.  There will be hostile action taken by regimists which will make this task difficult.

The danger is that the military is divided and there is sufficient dissent within and from police departments and the Reds that the Constitutionalist coup leaders cannot maintain control.  The strife will lead to a civil war of some leel.