the remaining SBs were used for occasional training flights until withdrawn and scrapped in 1948. China In July 1937, the Second Sino-Japanese War broke out. The Soviet Union signed the Sino-Soviet Non-Aggression Pact on 21 August 1937, and as part of this agreement, supplied large amounts of military equipment to the Chinese Nationalists, as well as deploying complete air force units, nominally manned by Soviet volunteers. An initial delivery of 62 SBs was made in September–October 1937, with combat operations by Soviet forces starting in December with attacks on Japanese ships on the Yangtze River. On 23 February 1938, to celebrate Soviet Army Day, Soviet SBs carried out a long range attack on Japanese airfields on Taiwan, claiming 40 Japanese aircraft destroyed on the ground. A further 60 SBs were delivered to China in early 1938, these being heavily used to attack Japanese forces during the Battle of Wuhan. Losses were heavy, forcing the Chinese SB units to be temporarily withdrawn from combat. The Soviet units operating the SB over China re-equipped with the Ilyushin DB-3 in 1939, allowing their SBs to be transferred to Chinese units, but the Chinese made limited use of these reinforcements. The Soviet Union supplied a further 100 SBs in 1941, just before it signed the Soviet–Japanese Neutrality Pact. The SB was gradually phased out of front-line operations against the Japanese with the delivery of more modern American bombers from 1942, being partly replaced by Lockheed Hudsons and B-25 Mitchells. Limited numbers of SBs continued in non-combat use, including operations against opium plantations, before being used against the Communists when the Chinese Civil War flared up in 1945, being finally withdrawn in 1946. Mongolia As well as the aircraft operated by volunteers against the Japanese over China, SBs were used in combat against the Japanese during the Fighting near Lake Khasan on the eastern sector of the China–Russia border in July–August 1938, one SB being lost. Fighting between Soviet and Japanese forces broke out again at Khalkhin Gol in Eastern Mongolia in May 1939. While SBs were not involved in the May air battles, where the Soviet forces received heavy losses, two Regiments of SBs were deployed to Mongolia in June, flying their first missions on 26 June. SBs were used heavily against Japanese forces when they attacked in early July. The Soviet SB regiments consisted of a mixture of early and later SBs, whose