Delta Green: An organisation which encourages middle-aged accountants and bureaucrats to drive vans filled with dynamite into the homes of white supremacists who worship the risen spirit of Hitler.
Once an official government agency charged with identifying and neutralizing preternatural threats against the United States, Delta Green was disbanded following a disastrous operation in Cambodia in 1969. Within a year, the leaders of Delta Green regrouped and continued the fight as an illegal conspiracy.
During World War II, Delta Green fought the Karotechia and came into contact with the British group PISCES. They have had some information about members of the German group surviving, but not enough to take them down. After the war, Delta Green and PISCES had intermittent contact until the former was disbanded in 1970.
From 1994 onwards, Delta Green is organized as a cell structure, with each cell consisting of three Agents. This may have changed in 2001 or soon thereafter. Delta Green after 9/11 has been a topic of speculation since the terrorist attacks.
Delta Green cells are named alphabetically (A through Z), with Agents code-named accordingly (e.g. Agents Roland, Richard, and Roy for Cell R). Cell hierarchy is ambiguous, except that the command cell of Delta Green is Cell A, consisting of Alphonse, Adam, and (possibly) Andrea.
Agents are the backbone of the organization. Generally aware of the illegal nature of Delta Green, they conduct the majority of its investigations with very little information or support from above. They must understand and exercise tradecraft.
Agents recruit Friendlies, people who are not informed about the true nature of the organization, and not qualified to take on heavy responsibilities. Many Friendlies are experts who've run into the Mythos in their own lives.