Beginning in the 1960s, vast quantities of obsolete and redundant SKS carbines from military reserve stocks were donated by the Soviet Union and the People's Republic of China to left-wing guerrilla movements around the world. The increasing ubiquity of the SKS altered the dynamics of asymmetric warfare in developing nations and colonial territories, where most guerrillas had previously been armed with bolt-action rifles. For example, the SKS served as one of the primary arms of the Viet Cong during the Vietnam War. The weapon type was encountered so frequently by the United States Armed Forces in Vietnam that captured examples were used by opposing force (OPFOR) units during training exercises designed to simulate battlefield conditions there as early as 1969. Captured SKS carbines were also prized as war trophies among individual US military personnel, and a number were brought back to the United States by returning veterans over the course of the Vietnam conflict.