Food for thought, excerpt from Martial Law Would Sweep the Country Into a Great Legal Unknown at https://www.theatlantic.com/ideas/archive/2020/03/martial-law-would-sweep-country-great-legal-unknown/608773/ [With some clear anti-POTUS bias.] Emphasis added.  During the Civil War, President Lincoln declared martial law with regard specifically to “rebels and insurgents” and certain other disloyal individuals. Although Lincoln limited the application of martial law (it did not apply to the general population), in 1866 the Supreme Court ruled, in a case called Ex parte Milligan, that his order to try such persons by military commission was illegal where civilian courts remained open and operating. In its decision, the Court laid out criteria for invocation of martial law:  If, in foreign invasion or civil war, the courts are actually closed, and it is impossible to administer criminal justice according to law, then, on the theatre of active military operations, where war really prevails, there is a necessity to furnish a substitute for the civil authority, thus overthrown, to preserve the safety of the army and society; and as no power is left but the military, it is allowed to govern by martial rule until the laws can have their free course.  And, the Court said, “As necessity creates the rule, so it limits its duration.” The Court warned of allowing this extraordinary measure under other circumstances: “Civil liberty and this kind of martial law cannot endure together; the antagonism is irreconcilable; and, in the conflict, one or the other must perish.” The Court apparently failed to consider the possibility that the courts might be closed by military order. In such a case, of course, no one would be left to judge the legitimacy of the military’s actions. The objective criteria set out by the Court—the presence of war, actual insurrection or invasion, effective closure of courts, displacement of civil authorities, and no alternative to protect the Army and society—left plenty of room for judgment about when martial law could be proclaimed. Yet they revealed the kind of extreme conditions that might justify its use.  More info at https://lawbrain.com/wiki/Martial_Law  ...Factual and non-biased; with good sourcing and "further reading"