The ZSU-23-4 "Shilka" is a lightly armored, self-propelled, radar guided anti-aircraft weapon system (SPAAG). The acronym "ZSU" stands for Zenitnaya Samokhodnaya Ustanovka (Russian: Зенитная Самоходная Установка), meaning "anti-aircraft self-propelled mount"; the "23" signifies the bore diameter in millimeters; the "4" signifies the number of gun barrels. It is named after the Russian Shilka River. Polish soldiers nicknamed it maszyna do szycia (sewing machine) due to the sound of firing guns and because of the similarity of the name "Shilka" to the Russian word "шить" (Russian pronunciation: [ʂitʲ], meaning "to sew"). It is also referred to by its nickname of "Zeus".
The previous Soviet self-propelled anti-aircraft gun (SPAAG), the ZSU-57-2, was armed with two 57 mm autocannons; it was aimed optically using a basic tracking and lead calculating system. The ZSU-57-2 was not particularly successful despite its very powerful autocannons; it could only carry a relatively small amount of ammunition, was inaccurate as it lacked radar and could not fire while on the move.
The ZPU series armed with 14.5 mm heavy machine guns carried on a towed mount for stationary, point air defence had a much higher rate of fire. The 23 mm version of this weapon system was known as the ZU-23-2, a towed mount carrying two 23 mm cannons.
The development of the ZSU-23-4 "Shilka" began in 1957 along with ZSU-37-2 "Yenisei" and the vehicle was brought into service in 1965, replacing all ZSU-57-2s in air defense units toward the beginning of the 1970s. The ZSU-23-4 was intended for AA defense of military facilities, troops, and mechanized columns on the march. Initially, tank regiments should have had the anti-aircraft artillery battalion of "Shilka" (consisting of two batteries, four ZSU-23-4s in each). At the end of 1960s, one battery was equipped with ZSU-23-4s and the other with ZSU-57-2s. Motorized rifle and tank regiment standard anti-aircraft batteries consisted of two platoons later (one platoon was equipped with four ZSU-23-4s and another with four mobile surface-to-air missile systems 9K31 Strela-1 or 9K35 Strela-10). The ZSU-23-4 combined a proven radar system, the non-amphibious chassis based on GM-575 tracked vehicle, and four 23 mm autocannons. This delivered a highly effective combination of mobility with heavy firepower and considerable accuracy. The ZSU-23-4 outclassed all NATO anti-aircraft guns at the time, and it is still regarded as posing a major threat for low-flying fixed-wing aircraft and helicopters.
The system was widely fielded throughout the Warsaw Pact and among other pro-Soviet states. Around 2,500 ZSU-23-4s, of the total 6,500 produced, were exported to 23 countries. The Soviet Union's successor states continue to manufacture and supply variants of the ZSU-23-4, notably the Ukrainian "Donets" and Polish "Biala" variants.
ZSU-23-4 units saw active service in the Yom Kippur War (1973) and other Arab-Israeli conflicts, the Iran–Iraq War (1980–1988), and the First Gulf War (1990). During the 1973 Yom Kippur War, the system was particularly effective against the Israeli Air Force. Israeli pilots attempting to fly low in order to avoid SA-6 missiles were often shot down by ZSU-23-4s as in Operation Doogman 5. During the Soviet-Afghan War ZSU-23-4 units were used widely and to great effect against mujahideen positions in the mountains, the ZSU-23-4's guns being able to elevate much higher than the weapons on BMPs, BTRs, T-55s, or T-62s. They were also used to suppress defensive positions around the presidential palace during the initial coup in Kabul at the start of the Soviet-Afghan war. The Russian Army used the ZSU-23-4 for mountain combat in Chechnya.