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 also 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. Mongolia In 1939 Polikarpov fighters were extensively used during the Battles of Khalkhin Gol fought around the Khalkha River in Dornod Province. The battles were fought during 11 May–16 September 1939, and involved more than 600 planes. When hostilities commenced, the only I-15bis in the area were 14 aircraft of 70th IAP. Their number increased in the following weeks: on 23 May, 35 I-15bis from 22nd IAP arrived from the Trans-Baikal region. However the Polikarpov pilots had been hastily trained and they suffered heavy losses against the more experienced Japanese. During this conflict, the Soviet Union and Japan lost more than 200 aircraft each. 10 aircraft were delivered to the Mongolian People's Army Air Force in mid-July 1939 and flight personnel were trained for rear air defence. Afterwards, they received more than 30 aircraft in March 1942. Spain The I-15 was used extensively in combat by the Republicans in the Spanish Civil War and proved to be one of the best fighter biplanes of its time. The Nationalists called the fighter "Curtiss", apparently believing it to be the Curtiss F11C Goshawk. The first batch of 25 Polikarpovs arrived in Cartagena, Spain, on 28 October 1936, with 15 pilots, led by future ace Pavel Rychagov. A few days later a further group of 10 pilots and 15 aircraft arrived in Bilbao. The Soviet pilots first went