X-2 Counterintelligence Branch

The Counterintelligence Branch, or X-2, often mistakenly referred to as the Counter Espionage Branch (espionage is only one part of the larger intelligence field), was the most secretive branch of the Office of Strategic Services (OSS), acting as that organization's counterintelligence (CI) capability. As a CI force, their primary mission was to find moles and double agents, and had their field branch units posted away from the facilities of any of the other OSS field branches. In that capacity, hunting for Axis spies embedded within the Allied force, but especially within OSS, X-2 had the power to veto operations of the Special Operations (SO) and Secret Intelligence Branches (SI) without explanation. X-2 established separate lines of communication for itself as a self-contained unit. By the end of World War II, the X-2 had discovered around 3,000 Axis agents. X-2 was led by James R. Murphy at OSS/London, who took over command from George Hunter White, who had been sent on a mission to Calcutta.

William Donovan modeled the Counterintelligence Branch on British Counter Espionage. With the creation of the X-2 Branch, the British insisted that it follow British security procedures to maintain the secrecy of Ultra.