The introduction situates the book within Cold War history, intelligence studies, and New Left scholarship, showing how newly available archives complicate long-standing assumptions about the FBI’s counterintelligence investigations. It argues that while the Bureau engaged in serious abuses through its dragnet surveillance and coercive tactics, it also possessed fragmentary intelligence suggesting real, if uncertain, foreign ties with some activists. The chapter emphasizes the ambiguities of counterintelligence work, where incomplete information could both justify and escalate investigations. It reviews alternative explanations for the FBI’s actions, rooted in interests, ideology, and a permissive institutional environment, but stresses the need to also account for what the Bureau believed it knew about foreign influence. By addressing evidentiary challenges, defining counterintelligence, and previewing the book’s structure, the introduction provides the framework for reassessing how U.S. agencies interpreted and responded to the New Left in the Cold War.