Regular army forces maintain law and order in support of civil authorities when directed by

The Department of Defense. This Department is headed by the Secretary of Defense. Examples of this from history would be when forces like the National Guard were called out to perform various tasks like guarding Little Rock High School and the Little Rock Nine, or the when they were sent to Kent State for the Vietnam protests, or the current help in enforcing and guarding the Mexican Border. Not all the occasions worked out. Their deployments are often highly contentious. 

Word of mouth.