The T-54 and T-55 tanks are a series of Soviet main battle tanks introduced just as the Second World War ended. The first T-54 prototype appeared in March 1945 and entered full production in 1947. It became the main tank for armoured units of the Soviet Army, armies of theWarsaw Pact countries, and many others. T-54s and T-55s were involved in many of the world's armed conflicts during the late 20th century.  The T-54/55 series eventually became one of the most-produced tank in military history. Estimated production numbers for the series range from 86,000 to 100,000. They were replaced by the T-62, T-64, T-72, T-80, and T-90 in the Soviet and Russian armies, but remain in use by up to 50 other armies worldwide, some having received sophisticated retrofitting.  During the Cold War, Soviet tanks never directly faced their NATO adversaries in Europe. However, the T-54/55's first appearance in the West in the 1950s spurred the United Kingdom to develop a new tank gun, the Royal Ordnance L7, and the United States to develop the M60 Patton.