For years now, amateur actors in period attire have gathered to re-enact key battles from the Civil War.

Last Saturday brought a cruel perversion of that public ritual, as throngs of Americans, also dressed in strange costumes, assembled in Charlottesville, Virginia, near a monument to Robert E. Lee to do battle with their fellow citizens. Plans to remove the statue drew protests from those on both sides of the issue, including extremists bearing swastikas and Confederate flags.

By day’s end, three people were dead. A woman who had come to protest the white supremacists at the gathering died when a self-professed admirer of Adolf Hitler drove into a crowd of those who opposed his views; 19 other people were injured. Two Virginia State Police troopers monitoring the melee from the air also died when their helicopter crashed.

The Civil War involved a central question of civil society. Would we be a nation bound by a common commitment to constitutional order, or a country compromised by the chaos of factionalism? The conclusion of that conflict, purchased by the blood and anguish of an America divided against itself, was supposed to make us whole once more.

But as recent days have demonstrated, Americans now face new threats to constitutional liberty.

The particulars have the power of a living nightmare, the regular features of our national life lying distorted as if in a dark dream. The automobile, once an American icon of freedom, has been appropriated as a tool of terror. Not since Sept. 11, 2001 — when airliners, instruments of international connection, became flying bombs — has something so good been twisted into something so terrible.

Beyond Charlottesville, civil society seems frayed. In Durham, North Carolina, protesters who disliked a Confederate statue took matters into their own hands and tore it down. In a matter of minutes, a mob became judge, jury and executioner regarding a piece of public property.

Such discord demands a renewed call to law and order, a mission made all the more compelling by Louisiana’s own recent struggles with the legacy of the Confederacy.

In New Orleans, after extended public debate, the City Council agreed with Mayor Mitch Landrieu that four monuments related to the Confederacy should come down. We sometimes criticized the process that prompted the removals, suggesting that it often seemed more driven by passion than principle — with Landrieu pushing a predetermined result rather than raising a range of possible solutions.

But what happened in New Orleans, though messy and mercurial, unfolded within the law, with the courts on hand to ensure that the city’s executive and legislative branches operated legally. Such protections must remain the refuge of any republic that hopes to remain free.