From 1965 onwards, the Soviet Union began supplying the Democratic Republic of Vietnam with large shipments of weapons.Tucker, Spencer C (2011). The Encyclopedia of the Vietnam War: A Political, Social, and Military History. ABC-CLIO. pp. 52. The 23 mm ZU-23-2 was, along with the 37 mm M1939, the most frequently encountered anti-aircraft gun in Vietnam. Given that 83% of the USAF losses came from ground fire, the ZU-23 was probably responsible for shooting down hundreds of aircraft.
