The 1st Special Forces Operational Detachment-Delta (1st SFOD-D) commonly known as Delta, Delta Force or the Combat Applications Group by the United States Department of Defense, is an elite Special Operations Force (SOF) and an integral element of the Joint Special Operations Command (JSOC). It is the United States' primary counter-terrorist unit.

Delta Force's primary tasks are counter-terrorism, counter-insurgency and national intervention operations, although it is an extremely versatile group capable of assuming many covert missions, including, but not limited to, rescuing hostages and raids. Delta Force conducts missions similar to those attributed to the British Special Air Service (SAS), on which it was originally modeled.

The initial concept of Delta was a direct result of numerous, well-publicized terrorist incidents that occurred in the 1970s. As the threat of terrorism was on the increase and the United States being particularly targeted by enemies abroad, the administration felt it needed a counter-terrorist capability.

Key military and government figures had already received briefing on a model for the unit. Charles Beckwith, a member of the US Army Special Forces had served as an exchange officer with the British Special Air Service (22 SAS Regiment) in the early 1960s. US Army Special Forces in that period focused on unconventional warfare, but Beckwith was impressed with the SAS direct action and counter-terrorism capabilities. He briefed military and government figures, who were also impressed with the SAS concept.

Beckwith had estimated that it would take 24 months to set up the unit. As the threat of terrorism was increasing, the administration needed a counter-terrorist capability until Delta became fully operational. The 5th Special Forces Group created Blue Light as a small contingent of men that would train for this mandate. Blue Light existed into the early 1980s until Delta Force was fully operational.

April 24, 1980, shortly after Delta had been created, 52 Americans were taken captive and held in the U.S. Embassy in Tehran, Iran. The unit was assigned to Operation Eagle Claw, with the goal to covertly insert into the country and recover the hostages from the embassy by force. The mission failed because of an overly complex plan, inadequate Special Operations Aviation training for the accompanying aircrews, a collision between a rescue helicopter and a refueling tanker aircraft, and mechanical problems that reduced the number of available helicopters from the initial eight to only five (one fewer than minimum required) before the mission contingent could leave the refueling site to stage for the attack.[citation needed]

After the failed operation, the US government reassessed its counter-terrorist capabilities and as a result created new units including the Navy's SEAL Team Six and the 160th Special Operations Aviation Regiment (Airborne), also known as the "Nightstalkers", particularly focused towards the type of mission like Operation Eagle Claw. The Joint Special Operations Command was also created to control and oversee joint training between the counter-terrorist assets of the various branches of the US military.

In 2006, Mark Bowden published, Guests of the Ayatollah: The First Battle in America's War with Militant Islam, which chronicles the events of the Iran hostage crisis. The book contains first-hand accounts of Delta Force's involvement in the failed rescue attempt. An accompanying piece on The Atlantic Monthly's web site contains pictures and interview videos from some of the participants.