The US nuclear command-and-control decision tree includes entire branches for scenarios in which civilian authority is uncertain: nonexistent, unreachable, contested, unverifiable, and/or incompetent. Little or nothing is publicly known about the criteria and procedures involved in switching to one of those branches. But I’m pretty sure that the president firing the Secretary of Defense while raving about imagined conspiracies, contesting the election, and threatening to never leave would meet those criteria. Feel free to disagree.