Antifa’s complexity was part of what Federal Bureau of Investigation Director Chris Wray tried to explain during the congressional testimony Mr. Biden cited in the debate. “Antifa is a real thing. It is not a fiction,” Mr. Wray said. “Trying to put a lot of these things into nice, neat, clean buckets” is “a bit of a challenge, because one of the things that we see more and more in the counterterrorism spaces [is] people who assemble together in some kind of mishmash, a bunch of different ideologies. All—we sometimes refer to it as almost like a salad bar of ideologies, a little bit of this, a little bit of that, and what they’re really about is the violence.” Mr. Wray vowed that “we are not going to stand for the violence” and said the FBI is investigating “anarchist violent extremists” and “their funding, their tactics, the logistics, their supply chains.”

Antifa’s lack of a central structure is what makes it effective at imposing disorder on American cities. Without leadership, no one can moderate the movement or prevent a protest from becoming a riot. If antifa were a conventional organization, the government could cripple it by bringing criminal charges against its leaders and financial backers. Instead, it can only prosecute low-level activists who commit street crimes. Even that often proves difficult, since antifa has adopted a “black bloc” uniform that makes it difficult to tell rioters apart. Instead of a hierarchy, law enforcement is now contending with a hydra.