For those concerned about a constitutionally unhinged presidency, Donald Trump’s decision at the end of October to deploy what has grown to be about 5,600 active military personnel to Texas and other southwest border states in response to a caravan of Central American migrants fleeing violence in their home countries was alarming. And that alarm only deepened on November 20 when the White House authorized those troops to use deadly force.

From the founding of the United States, deployment of the federal military within the country has been a source of deep concern, and a longstanding federal statute—the recently much-mentioned Posse Comitatus Act—makes it a crime for federal military personnel, except in narrow circumstances, to engage in domestic law enforcement. (Posse comitatus—literally “power of the country”—was defined at common law as all those 15 and older whom a sheriff could call for assistance in preventing civil disorder.) Nonetheless, precious little law—including no meaningful Supreme Court precedent—addresses the deployment of American troops domestically. The ongoing presence of the military in the southwest United States provides a useful opportunity to examine the little authority that exists.