The Republic of China Air Force (ROCAF) was officially formed by the Kuomintang (KMT) after the establishment of the Aviation Ministry in 1920. As tensions mounted between China and Imperial Japan in the 1930s, air units from the Chinese warlords, including those from the Guangdong Provincial Air Force, and overseas Chinese aviators, became integrated into the centralized command of the ROCAF, nominally as the Nationalist Air Force of China, and coordinating with the Second United Front to counter the Imperial Japanese invasion and occupation. Of all the Chinese warlord air force units to join the centralized Nationalist Chinese Air Force command, the Guangxi Clique was the last to unite, in November 1937; under the continued leadership of generals Li Zongren and Bai Chongxi, now serving in the KMT, they and their airmen would earn honorable recognition at the Battle of Taierzhuang. Operations During the Second Sino-Japanese War (1937–1945), which is often regarded as the beginning of World War II, the ROCAF participated in attacks on Japanese warships on the eastern front and along the Yangtze river and interdiction and close-air support during the Battle of Shanghai in 1937. Chinese sources estimated the Japanese could field approximately 600 aircraft (from a total of 1,530) against China's 230 combat-ready aircraft. As the Imperial Japanese Army Air Service and the Imperial Japanese Navy Air Service conducted mass-terror bombings against both civilian and military targets, the Chinese Air Force also planned raiding Japanese home island with the Martin B-10 bomber that was suited for the trans-oceanic raid. The initial lineup of Chinese frontline fighter aircraft included but not limited to the Curtiss Hawk II, Hawk III, Boeing P-26C/Model 281, Fiat CR.32 among others. These engaged Japanese fighters, attack/bombers and reconnaissance aircraft in many major air battles beginning from 14 August 1937, when Imperial Japanese Navy's Kisarazu Air Group raided Jianqiao Airbase with the schnellbomber strategy, but suffering heavy losses; "814" has thus become known as "Air Force Day". Chinese Boeing P-26/281 fighters engaged Japanese Mitsubishi A5M fighters in the world's first dogfight between all-metal monoplane fighters. A long-distance "bombing raid" over Japan on 19 May 1938 saw two Chinese Martin B-10 bombers led by Captain Xu Huansheng fly into Japan, albeit dropping only anti-war leaflets over the Japanese cities of Nagasaki, Saga, Fukuoka, among others cities. It was a war of attrition for the Chinese pilots, as many of their most experienced ace fighter