The University of California estimates it cost $600,000 to keep order last week when a conservative commentator made an appearance on its flagship Berkeley campus. In recent months, self-styled antifascists, or “Antifa,” have entered public consciousness by holding disruptive, sometimes violent protests in cities across the nation. After Antifa members disrupted a pro-Trump rally in Berkeley last month, Mayor Jesse Arreguin labeled the loose-knit group a “gang.”

As criminologists who have studied gangs, in the U.S. and abroad,...