Martial law has been declared at least 68 times in the United States. In the nine times it’s been declared since World War II, five of those were designed to counter resistance to federal desegregation decrees in the South.

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There is a ton of confusion around martial law, mainly because the U.S. Constitution doesn’t mention it and no act of Congress defines it. The U.S. Supreme Court has addressed it, but only a few times. The last time they did was in the case Duncan v. Kahanamoku (1946) and even then the decision was vague.

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1787’s interpretation is that under current law, the president lacks the authority to declare martial law. Congress may be able to authorize a presidential declaration of martial law, but this seems to be inconclusive. State officials do have the power to declare martial law, but their actions and decisions must adhere to the U.S. Constitution and are subject to review in federal court. When martial law – which is only justified when civilian authority has ceased to function or has become ineffective – is in effect, the military commander of an area has unlimited authority to make and enforce laws. However, civilians may not be tried by military tribunals if civilian courts are functional.

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One of the most important things to know is that military personnel can’t be used in surveillance or undercover operations, and they can’t be used as informants, investigators or interrogators unless the investigation is a joint military-civilian operation where the military has an interest in the case’s outcome.