A special mission unit (SMU), at one time referred to as a "tier 1" unit, is a designation for the United States military's most highly secretive and elite special operations forces. The term special missions unit is also used in Australia to describe the Special Air Service Regiment. Special mission units have been involved in high-profile military operations, such as the killing of Osama bin Laden and Operation Kayla Mueller, which resulted in the death of Islamic State (IS) leader Abu Bakr al-Baghdadi.
