The Zenitnaya Samokhodnaya Ustanovka (Зенитная Самоходная Установка) 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. 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.