Socialism is on the upswing right now. And whenever radicalism has been on the rise over the past century, that has also meant that something else has gone on the rise: state repression, often at the hands of the Federal Bureau of Investigation.

That certainly was the case in the New Left era. In their book A Threat of the First Magnitude: FBI Counterintelligence and Infiltration from the Communist Party to the Revolutionary Union — 1962-1974, Aaron Leonard and Conor Gallagher tell a shocking and even gripping story of how the FBI did everything it could to destroy leftist groups emerging from the ferment of the student, civil rights, and antiwar movements. The book is a follow-up to their first book Heavy Radicals: The FBI’s Secret War on America’s Maoists and is based on a huge number of documents (many of which are included in the book) obtained from the bureau on a variety of left groups and activists.

At times, A Threat of the First Magnitude reads more like true crime than leftist history. The laundry list of FBI tactics for disrupting radical groups is incredibly long, and the reader often feels hot on the trail of infiltrators of these organizations — even at the highest levels of their leadership — as Gallagher and Leonard put together the pieces of how and why the FBI disrupted them.