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.

In approaching this examination, it is important to recognize that domestic deployment of federal troops is permissible and appropriate in highly unusual circumstances, the Civil War being the most notable example. And to be sure, federal troops have been used domestically in several momentous situations, including the unconstitutional internment of about 120,000 adults and children of Japanese descent during World War II and the post-World War II enforcement of federal court orders to desegregate Southern schools. But what makes Trump's deployment action so disconcerting—and thus so worthy of scrutiny—is that he ordered the deployment days before a major federal election, seemingly for political purposes. Given Trump's often reckless use of his extraordinary presidential power, reasonable people could worry that the current military deployment might be a prelude to further use of the military for Trump's political purposes.

Constitutional and Statutory Framework

Discussion of the limits on presidential power to deploy troops domestically starts with the Constitution. At the outset, of course, Article II, §2 provides that “[t]he President shall be Commander in Chief of the Army and Navy of the United States,” and §3 provides broadly that “he shall take Care that the Laws be faithfully executed.” As for Congress, §8 of Article I includes the power to “provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions.” Finally, the Bill of Rights contains some provisions obliquely bearing on the issue, with the Third Amendment providing that “[n]o Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner prescribed by law” and the Second Amendment providing that “[a] well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.”

Congress has partially filled this sparse constitutional landscape with three pieces of legislation. First is the Insurrection Act, originally enacted in 1807 and now codified at 10 U.S.C. §§251-55, which expressly authorizes presidential law-enforcement deployment of federal troops as follows:

Whenever the President considers 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 may call into Federal service such of the militia of any State, and use such of the armed forces, as he considers necessary to enforce those laws or to suppress the rebellion.

Next is the better-known Posse Comitatus Act, originally enacted in 1878 in the aftermath of the Civil War and now codified at 18 U.S.C. §1385, which criminalizes certain military participation in domestic law enforcement as follows:

Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.

Finally, in 1981 Congress enacted the Military Support for Civilian Law Enforcement Agencies Act (now codified at 10 U.S.C. §§271-84) in an effort to facilitate cooperation between the military and civilian law enforcement in dealing with drug trafficking and smuggling while “maintaining the traditional balance of authority between civilians and the military” and not running afoul of the Posse Comitatus Act. Accordingly, the Military Support Act permits the Secretary of Defense to “make available any equipment …, base facility, or research facility of the Department or Defense to any Federal, State, or local civilian law enforcement official for law enforcement purposes.”

Judicial Treatment

Whether because of the sparseness of relevant constitutional and statutory provisions, the infrequency of deployment of federal troops for domestic law enforcement, or the limited role of the courts in military affairs, no meaningful judicial precedent exists addressing limits on the President's deployment authority and only limited precedent addresses any application of the Posse Comitatus Act.

Starting with the Supreme Court, it has never meaningfully addressed the Act. Perhaps the closest it has come was in 1972 in Laird v. Tatum, 408 U.S. 1 (1972), which involved an Army surveillance program of political activity across the country. The program grew out of urban riots in 1967, during which President Johnson had invoked the Insurrection Act to deploy federal troops to Detroit. As the court explained:

The system put into operation as a result of the Army's 1967 experience consisted essentially of the collection of information about public activities that were thought to have at least some potential for civil disorder, the reporting of that information to Army Intelligence headquarters at Fort Holabird, Maryland, the dissemination of these reports from headquarters to major Army posts around the country, and the storage of the reported information in a computer data bank located at Fort Holabird … . Some of the information came from Army Intelligence agents who attended meetings that were open to the public and who wrote field reports describing the meetings, giving such data as the name of the sponsoring organization, the identity of speakers, the approximate number of persons in attendance, and an indication of whether any disorder occurred.

The plaintiffs in Laird challenged the program as chilling their right to assemble and engage in political activity in violation of the First Amendment, but the Supreme Court held that they were not sufficiently injured to establish standing and thus dismissed the case without reaching the merits. In closing, however, the court offered what may be its fullest treatment of the limits on federal military activity domestically:

The concerns of the Executive and Legislative Branches in response to disclosure of the Army surveillance activities—and indeed the claims alleged in the complaint—reflect a traditional and strong resistance of Americans to any military intrusion into civilian affairs. That tradition has deep roots in our history and found early expression, for example, in the Third Amendment's explicit prohibition against quartering soldiers in private homes without consent and in the constitutional provisions for civilian control of the military. Those prohibitions are not directly presented by this case, but their philosophical underpinnings explain our traditional insistence on limitations on military operations in peacetime … . [T]here is nothing in our Nation's history or in this Court's decided cases, including our holding today, that can properly be seen as giving any indication that actual or threatened injury by reason of unlawful activities of the military would go unnoticed or unremedied.

As for the federal Courts of Appeals, there are fewer than 100 cases that so much as cite the Posse Comitatus Act, and none appears to challenge presidential domestic deployment of the military. Rather, federal appellate cases typically involve criminal prosecutions in which federal military officials have played some role in the underlying investigation. Though not addressing troop deployment, these cases nonetheless provide some judicial treatment of the Act.

An illustrative example is United States v. Johnson, 410 F.3d 137 (4th Cir. 2005), a case that arose out of a car accident on the Baltimore-Washington Parkway, which is on property owned by the National Park Service. The driver of one car—defendant Tyronski Johnson—was arrested following the discovery of a gun in the glove compartment. Out of concern that Johnson might be seriously injured and a suspicion he was under the influence of PCP, he was taken to a hospital, which drew a sample of his blood. Pursuant to a contract between the U.S. Park Police and the Army's Armed Forces Institute of Pathology, the toxicology analysis of Johnson's blood was performed at the Institute, which confirmed the presence of PCP. Johnson pled guilty but preserved his right to appeal the denial of a suppression motion, which was based in part on an alleged violation of the Posse Comitatus Act.

Before the Fourth Circuit, the government argued the Army's analysis of the blood sample did not violate the Act because, in the terms of the Act, it was “expressly authorized by … an Act of Congress” in the form of the Military Support Act from 1981. But the Fourth Circuit rejected this, holding that that statute authorized only the use of military equipment and facilities. As the Fourth Circuit explained, however, this does not mean that the statute “authorizes military personnel to perform 'searches' of blood in furtherance of misdemeanor DUI prosecutions.” Though the court did not order suppression because that remedy was unavailable except for in the most unusual of circumstances, it nonetheless found a violation of the Posse Comitatus Act. And in doing so, it noted the important values underlying the Act:

In sum, we interpret the Posse Comitatus Act and authorizing statutes in keeping with “the traditional American insistence on exclusion of the military from civilian law enforcement, which some have suggested is lodged in the Constitution.” Although performance of blood tests by military personnel for civilian prosecutions may not be an egregious encroachment on civilian law enforcement efforts, it is up to Congress to authorize such searches, and it has yet to do so.

The Caravan Deployment and Beyond

Three weeks after announcing the deployment of troops to southwest border states, the White House on November 20 executed two authorizing documents, a “Decision Memorandum” signed by the President and a memorandum—referred to as a “Cabinet Memorandum”—from White House Chief of Staff John Kelly to the Secretary of Defense, the Attorney General, and the Secretary of Homeland Security. Those documents authorized deployment of “appropriate numbers of the Armed Forces of the United States” and specified they could perform “military protective activities” that are “reasonably necessary … , including a show or use of force (including lethal force, where necessary), crowd control, temporary detention, and cursory search.” They also limited military action, providing that, “without further direction from the President,” troops could not “conduct traditional civilian law enforcement activities, such as arrest, search, and seizure in connection with enforcement of the laws.”

Notably and troublingly, neither document mentions any legal authority, nor does either explain how the deployment comports with the Posse Comitatus Act. This perhaps is unsurprising, as the Trump administration may be uninterested in acknowledging, much less confronting, legal limits on its power to deploy federal troops domestically. But this is a paramount concern for those worried about the President's aggressive use of his power for political purposes, and the prospect of a legal fight over future use of federal troops domestically and the Posse Comitatus Act is a chilling one.

Christopher Dunn is the associate legal director of the New York Civil Liberties Union. He can be reached at [email protected].