now looming in Europe, the supplies needed by the Chinese were barely trickling in, and the Chinese Air Force had to make do with the increasingly poor flying performance and maintenance problems of these Soviet-made fighters burning low-grade 65–75 octane fuel in the first few years defending Chongqing and Chengdu while massive formations of Japanese aircraft benefiting from great technological advancements and burning 90+ octane fuel were flying ever-faster and higher, almost beyond practical reach of the obsolescent Chinese fighter aircraft; the British Royal Air Force for example were supplied directly by their own aircraft industry while benefiting from tremendous improvements in their fighter aircraft speed, acceleration and high-altitude performance by upgrading from the standard 87 octane fuel to the "secret 100 octane" formula later in the Battle of Britain. Primary and auxiliary airbases used by the Chinese Air Force in the Chongqing and Chengdu defense sector included Baishiyi Airbase, Fenghuangshan, Guangyangba, Liangshan, Shuangliu, Suining, Taipingsi, Wenjiang, and Xinjin, among others. There were 18 primary anti-aircraft artillery batteries positioned around Chongqing from 1939 to 1942, not including the air force's anti-aircraft gun units assigned specifically for the defense of the airbases. 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