STAG is an elite paramilitary police force directed by Commander Cyrus Temple and his subordinate, Kia. It is under the direct command and supervision of the United States Government, and acts as a replacement for the SNG whenever the player reaches higher Notoriety levels (the Steelport PD and a minor SWAT presence still remain). STAG has their own, unique arsenal of futuristic weapons and vehicles.
STAG is deployed to Steelport following a devastating attack on the Hughes Memorial Bridge in Stilwater, perpetrated by the Luchadores. Senator Monica Hughes, wife of the late Alderman Richard Hughes (killed in Saints Row), gains unanimous approval from the US Senate to initiate "The STAG Initiative", essentially putting Steelport under martial law with the intent of eliminating the four gangs warring for control of the city.
STAG arrives in force during the mission "Gang Bang", aboard the aircraft carrier Thermopylae (which anchors offshore southeast of the 3 Count Casino), and begins deploying troops and mobile bases throughout the city. Initially relegated to a peacekeeping role in Steelport, STAG's role and actions intensify following the sinking of the Thermopylae by the 3rd Street Saints. The city is put under lockdown and STAG begins to actively combat the four gangs, focusing primarily on the Saints.