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. ...