In 1969, militant factions within both Students for a Democratic Society (SDS) and the Black Panther Party (BPP) began to form the United States’ first clandestine revolutionary urban guerrilla organizations: the Weather Underground and the Black Liberation Army (BLA). These groups carried out bombings, police ambushes, and other attacks throughout the country, prompting responses from the Federal Bureau of Investigation (FBI) and the administration of President Richard M. Nixon. Several historians have analyzed U.S. leftist guerrillas’ motives, and much has been written on FBI operations against the Black Power movement and New Left, including the Bureau’s covert counterintelligence programs (COINTELPROs) designed to “neutralize” these movements. Most of this scholarship has been one-sided, however, framing FBI activities as “state repression” without analyzing how state actors understood and responded to leftist violence. Drawing on declassified FBI documents and materials in the Richard Nixon Presidential Library, “Nixon’s War on Terrorism” revises this literature, explaining for the first time how domestic leftist guerrilla violence reshaped the FBI and American politics during the Nixon administration. War with domestic leftist guerrillas transformed the FBI’s surveillance practices, spawned the United States’ earliest institutions explicitly dedicated to combatting “terrorism,” and triggered a bureaucratic struggle between the Nixon White House and the FBI that played a critical role in fomenting the Watergate Scandal and Nixon’s August 1974 resignation. This dissertation examines how the FBI came to expand its surveillance of the U.S. Left and revive mail-opening, warrantless wiretapping, and break-ins—illegal spy techniques that Director J. Edgar Hoover had employed widely against the Communist Party after World War II but banned during the mid-1960s. This is a story of unintended consequences and conjuncture. Leftist guerrillas did not achieve their goal of sparking a socialist revolution, and the FBI was unsuccessful in its aim of preventing guerrilla violence. The Nixon administration was also unable to halt guerrilla attacks. But together—through their conflicts with one another—leftist guerrillas, FBI officials, and the Nixon administration triggered Watergate, the Church Committee, antiterrorism politics, and a crisis of popular legitimacy from which neither the Bureau nor the federal government have ever fully recovered.