Thai Jones has a PhD from Columbia University. His new book—More Powerful Than Dynamite: Radicals, Plutocrats, Progressives, and New York’s Year of Anarchy—tells the forgotten history of a protest movement that divided the city in 1914. It is published by Walker Books and is in stores now.

New York’s labor unions had a banner day planned for the First of May in 1914, with parades and celebrations scheduled across the city. Finnish and Bohemian socialists were congregating on the Lower East Side. Carpenters and cloak-makers would begin their march from Chelsea, while the United Hebrew Trades intended to start near Little Italy. By the afternoon, all these various strands would join in Union Square for an inspiring demonstration of working-class power in the nation’s greatest metropolis.

Yet a note of apprehension threatened to spoil this grand pageant of solidarity. Anarchists and members of the Industrial Workers of the World—radicals far to the left of the socialists and trade union members—had their own designs on May Day. The various factions had a long history of rancor and rivalry, and officials feared that a confrontation could flare into catastrophe.

All went well—at first. Mild weather ensured a tremendous turnout. For hours, 40,000 or so paraders marched with a “joyous spirit” through the city’s streets and filtered into Union Square. Then, around 5 p.m., a shouting match between anarchists and socialists degenerated into a terrifying melee. In a misguided attempt to restore order, hundreds of cops charged into the crowd, sparking pandemonium. “Clubs flew right and left,” the New York Times reported, with “police jumping over the bodies of prostrate women, men, boys, and even two babies,” until the “din of the stampede and the screams of women and children” echoed round the plaza. When order finally returned, bloodied victims lay everywhere, police and organizers faced widespread criticism, and another May Day had ended in disaster....