de opresso liber
The United States Special Forces were created by John F. Kennedy to develop a new type of guerilla warfare in the cold war. The lessons of Korea and World War II had given the hint that battles would no longer be fought and won by large troop movements; though this was slow to be accepted, no unit was equipped to deal with guerilla warfare (the Rangers are technically trained perform scouting and spearhead operations, rarely to engage in combat, when faced with which they are told to withdraw and call for reinforcements, and the Seals are trained for targeted rather than extended missions).
The military's response to this problem was to create the Special Forces. Their defined mission is counter insurgency; they are trained to work in small squads behind enemy lines and train native contingents in modern weapons and tactics, promoting United States foreign policy by using non-American soldiers.
The prime example was the use of the Montagnards in Vietnam; they were a native people, avoided by the South and North Vietnamese alike, living in the mountains and jungles and used to crossing the borders of Laos and Cambodia. Some of the greatest military successes of the Vietnam War were by Special Forces groups leading teams of Montagnards (can anyone say 'Mistah Kurtz?').
The trademark symbol of the SF is the green beret and dagger; the former to set them apart from the other army branches, the latter to symbolize stealth and precision.
Though I'm unsure about the unit numbers, each continent is assigned a Special Forces division; I believe South America is covered by the 7th, Asia by the 5th (I forget the rest).