The T-26 was a light tank used by the Soviet Union from the 1930s until World War II. It was based on the British Vickers 6-Ton tank and widely considered one of the most successful designs of the 1930s. The T-26 made-up the majority of the Red Army's armour force until late 1941, and saw a long history in the armed forces of various different nations around the world. For almost a decade the T-26 proved to be one of the best tanks in production, with a total of around 12,000 units produced. Success and failure in the Spanish Civil War, where it served as the most widely used tank, ultimately played a major role in influencing the Soviet doctrine of tank warfare in the late 1930s. The T-26 participated in German invasion of the Soviet Union in 1941 as one of the most numerous tanks in service, contributing to the defense of the Soviet Union. Although the T-26's reputation was marred by its abysmal performance during World War II, it was nevertheless the most important tank of the Spanish Civil War and played major roles during the Winter War and the Battle of Khalkhin Gol in 1939. Between its introduction and its retirement, the T-26 saw a great deal of modernization efforts between 1932 and 1941.