On April 20, 1914, members of the Colorado National Guard opened fire on a group of armed coal miners and set fire to a makeshift settlement in Ludlow, Colorado, where more than a thousand striking workers and their families were camped out. Today, the Ludlow massacre, which Caleb Crain wrote about in The New Yorker in 2009, remains one of the bloodiest episodes in the history of American industrial enterprise; at least sixty-six men, women, and children were killed in the attack and the days of rioting that followed, according to most historical accounts. Although it is less well-remembered today than other dark episodes in American labor history, such as the Triangle Shirtwaist Factory fire that claimed a hundred and forty-six lives, the Ludlow massacre—which Wallace Stegner once called “one of the bleakest and blackest episodes of American labor history”—changed the nation’s attitude toward labor and capital for the next several decades. Its memory continues to reverberate in contemporary political discourse.

In the summer of 1913, United Mine Workers began to organize the eleven thousand coal miners employed by the Rockefeller-owned Colorado Fuel & Iron Company. Most of the workers were first-generation immigrants from Italy, Greece, and Serbia; many had been hired, a decade prior, to replace workers who had gone on strike. In August, the union extended invitations to company representatives to meet about their grievances—including low pay, long and unregulated hours, and management practices they felt were corrupt—but they were rebuffed. A month later, eight thousand Colorado mine workers went on strike. Among their demands were a ten-per-cent pay raise, the enforcement of an eight-hour working day, and the right to live and trade outside the company-owned town. Many of the rights they sought were required by Colorado law but remained unenforced.

After getting evicted from their company-owned homes, the workers based their operations in makeshift tent cities surrounding the mines, the largest of which was the Ludlow camp. The Rockefellers responded by hiring a detective agency—comprised of “Texas desperadoes and thugs,” according to “Legacy of the Ludlow Massacre,” a sharply researched 1988 book by Howard M. Gitelman—who would periodically raid the camps, firing rifles and shotguns. In November, the state governor called in the Colorado National Guard at the company’s behest; the Guard’s wages were supplied by the Rockefeller family, and they helped to form militias whose members carried out sporadic raids and shootings in the tent cities.

The strike stretched on for months, and in April, 1914, John D. Rockefeller, Jr., appeared before Congress, where he framed the standoff as “a national issue, whether workers shall be allowed to work under such conditions as they may choose.” He balked at the possibility of allowing “outside people”—meaning union organizers—“to come in and interfere with employees who are thoroughly satisfied with their labor conditions.” The committee chairman asked Rockefeller whether he would stand by his anti-union principles even “if it costs all your property and kills all your employees.” Rockefeller replied, “It is a great principle.”

On April 20th, a day after Orthodox Easter, four militiamen brandished a machine gun at some of the striking miners. At some point, shots were fired—the accounts are predictably inconsistent as to who fired first—and a day-long gunfight ensued.

That evening, the National Guardsmen set fire to the Ludlow colony. Thirteen residents who tried to flee were shot and killed as the camp burned to the ground, and many more burned to death. Discovered among the ruins the following morning was a women’s infirmary, where four women and eleven children had sought to escape the fighting by hiding in a cellar-like pit. All the children and two of the women died. One survivor, Mary Petrucci, lost three of her own children in the infirmary. Years later, she recalled, “I came out of the hole. There was light and lots of smoke. I wandered among the ashes until a priest found me. I couldn’t feel anything. I was cold.”

News of the attack—and especially of the deaths under the infirmary tent—pulled the nation’s attention from the United States’ potential involvement in the Mexican Revolution. To many Americans, the massacre exposed the consequences of unchecked corporate might, and it roused the conscience of a country that had previously demonstrated impassive ambivalence toward organized labor. (Decades later, a song by Woody Guthrie captured the common sentiment of the event’s immediate aftermath: “We took some cement and walled the cave up where you killed these thirteen children inside / I said ‘God bless the Mine Workers Union,’ then I hung my head and cried.”)

Two days later, Congress convened to discuss the events at Ludlow, and to consider how the government might check martial power wielded by private industrialists. One senator, Iowa’s “radical Republican,” William Kenyon, decried the government’s ties to the violence, noting that “the Colorado Fuel & Iron Company, or the company controlling it, has certain of its bonds on deposit with the General Education Board of the Rockefeller Foundation, with which the Department of Agriculture of our Government seems to have been in partnership for some little time.” Another senator expressed a broader concern: “I fear that unless society can in some manner reconcile these troubled conditions as between capital and labor, Mexico is not the only country that will be torn by internecine strife.”

Rockefeller, for his part, released a memorandum in June, months after federal troops had been ordered to Colorado to quell the days of violent rioting that had followed the events of April 20th. “There was no Ludlow massacre,” he wrote. “The engagement started as a desperate fight for life by two small squads of militia … against the entire tent colony, which attacked them with over three hundred armed men.” He also offered a lengthy technical explanation of why the deaths in the infirmary were the result of inadequate ventilation and overcrowding, not of actions taken by “the defenders of law and property, who were in no slightest way responsible for it.”

Despite Rockefeller’s arguments, after Ludlow the Wild West era of company towns began to wane, and stricter labor laws began to appear on the books—and were even enforced. Support for unions reached an all-time high in the nineteen-thirties, as described by James Surowiecki in a 2011 article for the magazine. Yet, as Surowiecki also noted, the influence of trade unions, which supplanted company unions following the 1935 Wagner Act, has been declining for decades, as part of a general rightward shift in American politics which began in the sixties. Since the 2008 recession, there has been growing resentment for union members among non-unionized workers; in 2010, support for unions reached a historic low, according to a Pew poll.

Yet the struggle that Ludlow embodied—and that, historically, unions have taken up—is a contemporary one, even if unions are no longer playing as public a role. Today, some of the fiercest workers’-rights battles take place over government regulations that protect low-income workers’ access to Medicaid and other social services, and that buoy the federal minimum wage, which is currently far below its 1968 peak value. In her recently published autobiography, Senator Elizabeth Warren wrote that “Big corporations hire armies of lobbyists to get billion-dollar loopholes into the tax system and persuade their friends in Congress to support laws that keep the playing field tilted in their favor.” In this, she sounds almost exactly like the Republican senators who, in the days after Ludlow, worried about Colorado Iron & Fuel’s deep government influence.