The United States has stationed a substantial contingent of troops to defend South Korea. There are approximately 28,500 U.S. military personnel stationed in South Korea, most of them serving one year unaccompanied tours. The U.S. troops, which are primarily ground and air units, are assigned to USFK and mainly assigned to the Eighth United States Army of the U.S. Army and Seventh Air Force of the U.S. Air Force. They are stationed in installations at Osan, Kunsan, Yongsan, Dongducheon, Sungbuk, Camp Humphreys, and Daegu, as well as at Camp Bonifas in the DMZ Joint Security Area.