It is not possible to provide a definitive answer to the question of the “Military’s most secret unit,” as such information is classified and not publicly available. In a thread on Quora, a retired US Army Special Forces officer and enlisted soldier, who has served at various levels and locations, offered some insights into the subject. It is worth noting that the Central Intelligence Agency (CIA) is not affiliated with the US Department of Defense and therefore is not a military unit. Any discussions about CIA units and missions should be considered speculative at best.
The officer also defined “most secret,” which goes against the question’s premise because it asks for the “Military’s most secret team.” He stated that “most secret teams are most probably.” However, this is not a factual, evidence-based answer but rather a conjecture. It is also worth noting that the term “underhanded” is not a military term; therefore, its meaning is unclear.
In discussing some of the more well-known units in the Special Operations Forces (SOF) community, the officer noted that both Delta Force and US Navy SEAL Team 6 are not considered intelligence teams. Delta Force has never been referred to as a team, while SEAL Team 6 has been referred to as a team due to its original name. It is important to understand the differences between the terms “Direct Action” and “Assault” as they are used in SOF missions. “Direct Action” missions do not include hostage rescue, a separate mission. “Direct Action” is not interchangeable with “Assault,” and there is no such mission as an “Assault” mission in US SOF.
Many of the answers in the thread referenced the ISA (Intelligence Support Activity). However, it is worth noting that they are not a military unit but rather falls under JSOC (Joint Special Operations Command) and are not a Tier 1 Special Mission Unit. They have been known by the identifier “GREY FOX” among others in the past 20 years.