The full name of this group is the 1st Special Forces Operational Detachment-Delta. In addition to counter-terrorism operations, Delta Force can also take part in hostage rescue, raids, reconnaissance and less covert direct action operations. The group was formed in 1977 as a result of an increasing number of high-profile terrorist operations. Since then it has been composed largely of soldiers who have served in US special-forces like the Green Berets or Rangers. To be considered for training, potential candidates must be male, at least 21 years old, score highly on an aptitude test and be between the rank of corporal and master sergeant. A series of grueling physical and mental tests follow with the aim of weeding out the weakest. Allegedly, this testing means less than 1 in 10 make it through to the 6 month-long training course. Delta Force operations remain highly guarded secrets but you can bet they are in the vanguard of any US-led operation.