Refers specifically to the U.S. Army enlisted rank of Specialist (E-4). Not a general term for a specialist in other contexts.
彼は米陸軍の特技兵として勤務している。
He serves as a specialist in the U.S. Army.
Compound of 特技 (special skill) and 兵 (soldier), likely a direct translation of the U.S. Army rank 'Specialist'.