Prior to the 2016 election, private militia units trained for the event of a Hillary Clinton win. The Second Amendment people feel very strongly about the individual right to keep and bear arms. As did the Founding Fathers. The president warns of a "civil war like fracture" if he is removed from office.
Further revelations about Trump's communications with foreign leaders regarding Biden come out, convincing not only the House to impeach but the Senate to convict. The Senate convicts with a secret ballot to avoid political fallout for individual members. President Trump refuses to concede. He calls on the military and state National Guard units to defend him; this is to no avail, as their leaders refuse cooperation.
With his back to the wall, the president exercises his legal authority to call out the unorganized militia as its commander in chief. He reasons that the execution of the laws of the United States is being obstructed as a result of an impeachment predicated on neither a high crime nor a misdemeanor; and in fact, the impeachment is predicated on pretextual and malicious plots.
Many militia members refuse to obey the call, but many, able to furnish their own arms, supportive of the President and more or less geographically unified, answer the call. Thus begins the Second Civil War.