Members of the Arizona National Guard receive instructions last week at the Papago Park Military Reservation in Phoenix. (Caitlin O’Hara/AFP/Getty Images)
Kevin Adams is a specialist in the study of war and society in the United States, with a particular emphasis on the late nineteenth-century U.S. Army.

President Trump’s announcement this month that his administration was considering deploying the military to the U.S.-Mexico border touched off heated discussion about the legality of such a move. Critics say that it would violate the 1878 Posse Comitatus Act, which bans the use of active-duty military as domestic law enforcement. They say fidelity to the tenet of civilian control over the military is critical to upholding our founding principles.

Unfortunately, much of what Americans think they know about posse comitatus is wrong. While Trump’s actions may, in fact, be legally dubious, the army can be used for civil policing — as it always has.

Far from violating a founding principle of the republic, the use of the military as law enforcement was common in the 19th century, a function of wide-open spaces, small populations and underdeveloped institutions. “These realities,” Michael Tate says in “The Frontier Army in the Settlement of the West,” “left only one other legally constituted body with enough manpower and proper mandate to fill the enforcement void — the U.S. Army.” Within a generation of American independence, helping to police the frontier had become one of the U.S. Army’s primary missions.

After 1878, however, this state of affairs more or less vanished, despite the persistence of frontier conditions. It vanished because, as popular opinion tells us, the 1878 Posse Comitatus Act ended such practices. This widespread conclusion only has one small problem: There is no such thing as the 1878 Posse Comitatus Act.

Instead, a mere paragraph attached to the 1878 Army Appropriations Act addressed posse comitatus. It is a meaningful paragraph — for all its brevity, it serves as the foundation of our current posse comitatus policy — but its inclusion in an appropriations bill was extremely unusual for the time and speaks to why the story of posse comitatus is far more complicated than modern commentators realize.

The story of the 1878 “law” has far more to do with the Civil War and free African Americans than with the frontier and the settling of the West. Establishing order and protecting free people in the South after the war required an active and expansive military presence in the former Confederacy.

The Third Enforcement Act of 1871, Congress’s legislative riposte to waves of violence unleashed against African Americans and their white Republican allies by Southern Democratic white supremacists, granted the federal government the ability to — at the discretion of the president — use military force to protect constitutional rights when the state government proved unable or unwilling to do so.

Knowing that they could not challenge the military might of the army, Southern Democrats instead tried to use political means to weaken this enforcement power. As their numbers increased in the House of Representatives during the 1870s, they worked to reduce the size of the Army and hold its budget in check.

Restraining the legal ability of the military to intervene in civil society — for example, to be stationed at polling places on Election Day — would have served as another way to forestall renewed federal action to carry out the promises of congressional Reconstruction. But any “Posse Comitatus Act” introduced in the late 1870s would have been dead on arrival with a Republican president and Senate. (President Rutherford B. Hayes disclaimed any need for renewed military intervention in the South but simultaneously insisted that the executive retain the power to initiate said interventions.)

Facing this roadblock in their quest to roll back Reconstruction, Democrats turned to legislative hardball. Without consulting Republicans, they inserted clauses such as the posse comitatus ban into ordinary appropriations bills for federal agencies, thereby holding government funding hostage until they got their way. Republicans held firm for some time, resulting in a stalemate during which the U.S. Army was not paid for the better part of a year. In June of 1878, however, they accepted the posse comitatus clause to restore a functioning government.

Importantly, Republicans gained one key concession: The clause now included Republican language allowing the military to serve as domestic law enforcement when “expressly authorized by the Constitution or act of Congress.” Within a few days, the U.S. Army disseminated a general order recognizing nearly 20 statutory exceptions to the new prohibition, as well as an overriding constitutional principle — that Section IV, Article IV of the Constitution, which promised protection against invasion or domestic insurrection, justified the use of the military as domestic law enforcement.

One of those statutory exemptions would prove powerfully important, allowing — and perhaps requiring — the president to use federal military force to suppress widespread violations of civil rights either sanctioned by state authorities or beyond their power to control. In a supreme historical irony, by forcing Republicans to spell out the exemptions to posse comitatus, Democrats had essentially codified an assertive Republican approach to civil rights enforcement — the exact thing they were trying to prevent.

Nearly 80 years later, this “success” helped seal the fate of Jim Crow in the states of the former Confederacy. In 1957, the Eisenhower administration, citing an exception to the posse comitatus ban first enumerated by Gen. William T. Sherman in 1878, deployed the 101st Airborne Division to Little Rock to enforce court-mandated school integration. What Eisenhower did then, any president — before or since — could have done whenever “state authorities” allowed discriminatory practices to unfold in their jurisdictions.

This history reveals that the military can, in fact, legally serve in a law enforcement capacity. But before Trump backers triumphantly dismiss claims that the president can’t send troops to the southern border as “fake news,” there is a catch. None of the critical exceptions carved out by the Army in 1878 spoke to the sort of border security envisioned by Trump (the closest one involved the power of the military to enforce state-crafted disease quarantines at ports of entry).

Consequently, there is no obvious legal path to employ the active-duty military on our Southwestern border. The 19th-century statutory exceptions to posse comitatus mostly speak to 19th-century concerns, which did not include the subject of border security.

Probably the best legal argument the president could make is that the movement of people across the border constitutes an “invasion” the federal government would be duty-bound to repel given the language of Article IV, Section IV of the Constitution. This would be a highly strained argument (for one thing, none of the states directly affected has deployed its forces to combat the purported invasion). But it is important to remember that in periods of great crisis — real or imagined — strained arguments sometimes win the day.

In short, as the complicated tale of posse comitatus in the United States suggests, the republican institutions and values that defined the United States since its founding are always being negotiated and remade. Our choices are the only things that preserve them. Let us choose wisely.