In the weeks leading up to the midterm elections, President Donald Trump railed against the threat posed by an “invasion” of migrants making their way toward the southern U.S. border. Offering no evidence, the president claimed that “criminals and unknown Middle Easterners are mixed in” to the migrant caravans. In October, the president deployed more than 5,000 regular Army troops to the southern border, supplementing state-deployed National Guard soldiers already there. Now that the midterm elections are over, some expected that the manufactured fear of a crisis at the border would dissipate, if not vanish. No such luck. In an extraordinary “Cabinet order” signed on November 20 by White House chief of staff John Kelly, President Trump authorized active duty soldiers to perform functions in defense of border agents – including use of lethal force – that challenge the longstanding statutory presumptions against military involvement in domestic law enforcement enshrined in the Posse Comitatus Act (PCA).

Americans have embraced missions for the military at home with caution. We understand the value of a highly disciplined, well-equipped, experienced fighting force to defend against foreign invaders or to support civilian authorities overwhelmed by domestic violence or natural disasters. Sometimes U.S. troops operating on home soil have done what no other government entity could — saving thousands of lives and avoiding huge property losses, as when Hurricane Katrina struck New Orleans.

At other times, soldiers have threatened the very interests they are sworn to protect — civil liberties, representative government, and the rule of law. During World War II, for example, troops were used to round up and intern 120,000 Japanese-Americans. In the Vietnam War era, the Army illegally compiled personal dossiers on 100,000 Americans opposed to the war, in what the Church Committee called the military’s “worst intrusion” into American life. Military excesses in civilian life have usually not been the product of decisions by rogue officers or renegade soldiers. The military has almost always been respectful of civilian authority. But our civilian leadership has on occasion employed the military for political purposes in ways that violate civil liberties.

Kelly’s November 20 memorandum authorizes troops to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary” to protect border agents, including “a show or use of force (including lethal force, where necessary), crowd control, temporary detention, and cursory search.” It asserts that “credible evidence and intelligence” asserts migrant caravans moving toward the southern border “may prompt incidents of violence and disorder that could threaten” Customs and Border Protection and other government personnel and “prevent them from performing the Federal functions necessary to secure and protect the integrity of the southern border.”

The memorandum authorizes the Secretary of Defense to order deployed regular military to perform the activities quoted above, while in the next sentence it forbids soldiers from conducting “traditional civilian law enforcement activities, such as arrest, search, and seizure in connection with the enforcement of the laws.” Secretary James Mattis is directed to consult with Secretary of Homeland Security Kirstjen Nielsen on this mission, and with Nielsen and the Attorney General prior to withdrawing military personnel. Secretary Mattis defended the order on November 21, stating that “we don’t have guns in their hands right now. . . . I will determine it based upon what DHS asks for and a mission analysis.” In response to a follow up question about the use of force, Mattis said “they’re not even carrying guns, so just relax. . . . Don’t worry about it, OK?”

What about the “crowd control, temporary detention” and “cursory search” permitted by the order? Secretary Mattis responded to a question about involvement in law enforcement this way: “We do not have arrest authority. Detention, I would put it in terms of minutes. . . . [We would stop an assault on a CBP agent] and deliver them to a Border Patrol man, who would then arrest them.”

Since 1878, the PCA and its contemporary service branch regulations prohibit the military (except the Coast Guard) from enforcing domestic laws unless authorized by Congress or the Constitution. The default prohibition does not apply to National Guard troops acting under a governor’s authority. Because immigration and border enforcement is a federal law enforcement mission, state-deployed National Guard troops likewise cannot participate in direct law enforcement activities at the border. Earlier this year, National Guard troops were deployed to the southern border under the command of border state governors. Those forces were specifically prohibited from conducting arrests and could not use surveillance equipment pointed toward Mexico. Presidents Barack Obama and George W. Bush similarly deployed National Guard troops to the border to combat drug smuggling and related violence. The drug operations are specifically authorized by federal statute.

The Constitution’s plan for domestic security is convoluted and requires too lengthy an explanation for this essay. In brief, the federal government has the authority to deploy federal military forces to an incident in a state only if the crisis is especially grave. Under a series of federal statutes originating in 1792 and now generally known as the Insurrection Act, the President may unilaterally deploy federal military forces in a state under limited circumstances – general insurrection and invasion – and where necessary to execute federal laws, subject to a number of pre-deployment conditions. The Insurrection Act overcomes the statutory presumption against military participation in law enforcement enshrined in the PCA.

In theory, President Trump could have invoked the Insurrection Act, overcome the PCA presumption, and followed the statutory procedures to carry out the deployment. (The Insurrection Act was last invoked by President George H.W. Bush in 1992 to send federalized California National Guard troops, as well as active duty soldiers from Fort Ord and Marines from Camp Pendleton, to Los Angeles to help control rioting in the wake of the Rodney King trial verdict.) Prior to issuing such an order, the President must find that “unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State … by the ordinary course of judicial proceedings.” He must then issue a proclamation ordering “the insurgents or those obstructing the enforcement of the laws to disperse.” On the basis of the reported facts, it is hard to see how the required finding could be made at this time.

The best legal argument for the Cabinet order is to suspend disbelief and accept its claim that there is “credible evidence and intelligence” that migrants moving toward the southern border “may prompt incidents of violence and disorder.” If CBP personnel are in danger and cannot reasonably perform their border control and federal law enforcement tasks, then the President as Commander in Chief may act with a species of his defensive war powers – repelling an attack on the United States or its personnel. The scope and limits of this power are uncharted in circumstances like these.

Absent some version of this constitutional authority, the Cabinet order authorizes active duty military to act in violation of the PCA. The limited judicial record interpreting the PCA suggests that the legal line has been crossed if the military engages in activities that are “regulatory, proscriptive, or compulsory.” Temporary detention and cursory search certainly meet the test. Of course, the PCA is a criminal law and requires federal prosecution to enforce its terms. There have been no reported prosecutions of the PCA.

An alternative legal rationale for the Cabinet order may be found in DoD regulations providing “immediate response authority” for a local commander to respond quickly, upon the request of a civilian official, without relying on the Insurrection Act or waiting for approval from the chain of command in order “to save lives, prevent human suffering, or mitigate great property damage.” The regulation stipulates, however, that the response may not include law enforcement actions that would violate the PCA. This authority was cited when the Army furnished medivac aircraft, ambulances, bomb detection dog teams, and various personnel to assist civilians following the Oklahoma City bombing in 1995.

Another regulation provides for “emergency authority” to act without either presidential approval or a request from civilian officials to “quell large-scale, unexpected civil disturbances” that threaten “significant loss of life or wanton destruction of property,” in order to “restore governmental function and public order,” or to protect “Federal property or Federal governmental functions.” Neither regulation would support the use of force or military participation in law enforcement at the southern border at this time. The regulations do not cite their authority. Most likely they rely on residual constitutional powers of the President mentioned above. If a circumstance arose like one imagined by Secretary Mattis, soldiers would act defensively and lawfully to protect CBP agents, but unless conditions change dramatically the conditions for immediate response or emergency authority do not exist.

More important is what the Constitution, Posse Comitatus Act, and other federal laws represent – a longstanding legal norm disfavoring military involvement in domestic affairs except in dire circumstances. It is no exaggeration to say that avoidance of military involvement in civil society is part of our cultural heritage. Let’s hope that Secretary Mattis’ cool head prevails in the days ahead.

Photo of U.S. Army troops deployed to the U.S.-Mexico border, by Tamir Kalifa/Getty Images.