During the Second World War the P4 Division of ONI was enrolled into the Office of Strategic Services as a psychological warfare unit. As part of the OSS, P4 was given a new security classification - Delta Green. Very swiftly use of the classification became the de-facto name of the P4 Division within the OSS, and then within the P4 division itself.
History
The P4 Desk was founded in 1917 to investigate unnatural occurrences, but did not truly take off in its mission until the Innsmouth Raid in 1927. During the raid two-hundred and nine half human creatures were captured by P4 and a plethora of unnatural artefacts were recovered for study. P4 quickly became aware of the scale and direness of the threat posed by these Deep Ones and began a covert counter-occult campaign against them. In mid-1941 the US intelligence community was rebuilt with the establishment of the Office of the Controller of Information (COI). The P4 Division was transferred from ONI to the COI, and then to the OSS in mid 1942.
The activities and structure of the OSS was classified to the highest level, especially due to the organisation's involvement in campaigns of sabotage, espionage and assassination directed against the Axis powers. In order to become a member of P4 an individual had to acquire the 'Delta Green' security clearance, marked by a small green triangle on their personnel file. Over time the unit began to be identified more by this clearance than by their official name, calling themselves 'OSS-Delta Green'.
After the OSS was dissolved in 1945 the Delta Green clearance level remained in use. When Delta Green was reconstituted as an independent organisation, their operations remained classified under the Delta Green clearance. In order to be a Delta Green agent, a potential recruit had to reach Delta Green clearance.
Following the dissolution of Delta Green in the aftermath of Operation OBSIDIAN the Delta Green security clearance was officially retired. Every personnel file that sported the green triangle had it unceremoniously removed, though many of these ex-Delta Green decided to continue the legacy of the now-dissolved organisation by continuing the fight on their own terms.
In 2002 after the collapse of the Accord, the Delta Green security clearance was officially reactivated under the direction of the much reduced MAJESTIC-12. The reactivation of the clearance resulted in many of the veterans of Delta Green signing onto The Program. The activities of the modern Program remain classified to the level of Delta Green.
Delta Green Clearance
An individual who achieved a clearance level of Delta Green could be identified by the addition of a small green triangle to their personnel file. This symbol marked them as a 'friendly' to Delta Green, someone who could be trusted to assist in the organisations covert operations, though they were deliberately not informed of the true scale of the threat they were facing. Following the dissolution of Delta Green these green triangles were removed from the file of every friendly.