much of which came by way of French Indochina and processed locally; locally sourced oil came by way of the Yumen Laojunmiao oil wells in Gansu province beginning in the summer of 1939, which produced 90% of the native petrol (250,000 tons) for China during those war years. The Japanese warned foreign delegations in Chongqing to avoid being hit as collateral damage in the massed attacks by moving to pre-defined "secure areas" which would be exempt from bombing; a large Nazi flag was emblazoned on the roof of the German embassy in Chongqing, but was still hit by a Japanese air raid. Following the defeat of France by Germany in June 1940, the French Vichy Government submitted to the demands of the Japanese – allowing Japanese troops to conduct cross-border raids into Yunnan province, and the stationing of Japanese army air units at three airbases in French Indochina (now Vietnam), including Lạng Sơn. On 10, 12 and 16 June 1940, the Japanese raided Chongqing with 129, 154 and 114 bombers on these days respectively, while the Chinese I-15 and I-16 fighter squadrons engaged these attacks shooting down 13 of the raiders, perhaps more importantly, forcing bombers to miss their targets, although these disrupted flights of bombers may have evaded the Chinese fighters from their primary targets, and diverted out to drop bombs on secondary targets, including other large population and industrial areas such as Ziliujing and Luxian. The Chinese would be dealt with a serious blow a few weeks later in July 1940 when the British yielded to Japanese diplomatic pressure and closed the Burma Road, which was China's primary lifeline for material and fuel needed in the defense of Chongqing and Chengdu. On 11 August 1940, 4th PG CO Maj. Zheng Shaoyu personally selected a team of five of his top fighter pilots, including ace fighter pilot Capt. Liu Zhesheng and future ace Lt. Gao Youxin to deploy a new weapon in an experiment to help with the dispersal of, and attacks against, the massive Japanese bomber formations: multiple air burst bombs developed by combat flight instructor Yan Lei of the Central Air Force Academy at the Wujiaba Airbase. At 13:56 hours that day, 90 G3M bombers appeared in two waves, and Maj. Zheng's team, with ample preparations, maneuvered high and above the approaching bomber formation with precision flying while the Japanese aircrews observed the unusual movements of the