As the Chinese government and military forces fell-back and engaged the Japanese advances in new frontline operations in the beginning of 1938 at Wuhan, Taierzhuang, Guangdong and other main battle lines, the Imperial Japanese Army Air Force began harassing the deep-rear with exploratory strikes against the anticipated national fortress of Chongqing, while the Imperial Japanese Navy Air Force was hitting hard at the main battle-front of the temporary wartime-capital Wuhan. The exploratory strikes on Chongqing began on 18 February 1938, and continued sporadically at relatively low intensity until the commencement of ""Operation 100"" on 3 May 1939, when airbases of newly-captured territories, specifically in Hubei province including Wuhan, allowed the Japanese to establish airbases and logistics to stage a sustained campaign of massively coordinated "joint-strike" operations with the Imperial Japanese Navy Air Service heavy bomber units. The introduction of the Mitsubishi A6M "Zero" fighter in 1940, the most advanced fighter aircraft produced at the time, ensured the Japanese practically complete air supremacy. U.S. support for China had begun in earnest in 1941 following the Japanese invasion of French Indochina, with the oil and steel embargo against Japan, and consequently, the Japanese raid on Pearl Harbor; beginning in 1942, barrels of 100-octane avgas began to increasingly trickle into China through the cross-Himalayan air-route known as "The Hump" - mostly destined for United States Army Air Force (USAAF) operations - and the Chinese Air Force were provided with new American Republic P-43 Lancer pursuit planes, which in theory with its very fast high-altitude performance and good firepower from its four 50 cal (12.7 mm) M2 Browning machine guns, was quite the upgrade the Chinese Air Force needed to effectively hit back at the Japanese raiders, but it was proven to be extremely unreliable and may have killed more of its own pilots than the enemies, including veteran pilot and 4th PG CO Major Zheng Shaoyu whose P-43 caught fire during a ferrying flight back to combat operations in China and was killed in the ensuing crash.