When does the military have the right to step in?

https://www.newsweek.com/exclusive-inside-militarys-top-secret-plans-if-coronavirus-cripples-government-1492878

NORTHCOM stresses in almost everything it produces for public consumption that it operates only in "support" of civil authorities, in response to state requests for assistance or with the consent of local authorities. Legally, the command says, the use of federal military forces in law enforcement can only take place if those forces are used to suppress "insurrection, domestic violence, unlawful combination, or conspiracy." A second test also has to be met, that such disturbances "hinders the execution of the laws of that State, and of the United States within the State," that is, that the public is deprived of its legal and constitutional protections. Local civil authorities must be "unable, fail, or refuse" to protect the civilian population for military forces to be called in, Pentagon directives make clear.