In August 1937, the Chinese Kuomintang Government signed a non-aggression pact with the USSR. And, in autumn of the same year, the Soviet Union commenced to ship I-15s as a part of a programme of military aid to the Chinese Air Force (CAF) in its defensive war against Japan. More than 250 Soviet pilots volunteered to fly the 255 I-15s supplied to China in autumn 1937. By 1939, the total number of Polikarpov biplanes delivered to CAF reached 347 I-15/I-15bis. The I-15bis saw a great amount of action in the Soviet–Japanese border conflicts along the China–Mongolia border. From 1938-1941, I-15s in the Nationalist Air Force of China, fought many major battles, and skirmishes against invading and occupying Japanese forces, including the Battle of Taierzhuang, the Battle of Wuhan, the Battle of South Guangxi, the Battle of Chongqing-Chengdu, etc. The tough biplane was quite clearly outmatched during the debut dogfight against the new Mitsubishi A6M Zero fighter over Chongqing on 13 September 1940, although I-15bis pilots Maj. Zheng Shaoyu, Lt. Gao Youxin, and Lt. Xu Jixiang were able to target and damage some of the Zeroes, with Lt. Gao believing he had shot one down (all Zeroes returned to base in Wuhan, with four Zeroes suffering some damage); all three of those pilots survived the battle.