The US Army's Delta Force has been at the forefront of the US military force for years.
But who are they and what do they do?
Formed in the late 1970s, the Delta Force was created in the image of the UK's Special Air Service (SAS).
The Unit, as it is often referred to, was formed by Charles Beckwith – a US Army officer and veteran of the Vietnam War who served with the SAS.
However, it took 10 years to persuade the US top brass that an American equivalent was needed before a number of terrorist attacks in the 1970s led to the formation of Delta Force as the first full-time counter-terrorism unit in the US.
Now, the unit specialises in counterterrorism and hostage rescues and has been involved in all of the US operations since it was created, including the Iran hostage crisis.
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Considered the best of the best, Delta Force's selection process is one of the most challenging in the US military.
The majority of Delta Force operations remain classified although there are a few instances where details have emerged – most notably Operation Iraqi Freedom and the subsequent capture of Saddam Hussein.
In order to qualify, an individual must be in the Army, aged 21 or over and be Airborne qualified.
Recruits must also be able to secure Secret Security clearance, have no disciplinary action on their military file and have a minimum of two-and-a-half years of obligatory service remaining.
If you hit all the criteria, the selection for the Delta Force is split into three sections – with training taking part in the mountains of West Virginia.
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In the first phase, a cadre, made up of Delta Force operators, make sure candidates are acquainted with the course and go through basic physical training, land navigation and other assessments.
Candidates must also complete individual marches and marches led by the instructor – there are two choices for candidates, keep up or quit.
Next up is the 'Stress Phase', where candidates remain on the field, without returning to barracks.
Every morning, they are sent co-ordinates and must get there with just a compass and a map.
Candidates cover an average of 12 to 18 miles a day throughout the course.
They are also not allowed to communicate with each other at all, with instructors emphasising that selection is an individual effort.
The last phase of training is a timed 40-mile march.
If completed, candidates are then grilled by the Delta Force commander to see if an individual would be a good fit for Delta Force.