pilots, such as Lieutenant , Colonel Kao Chih-hang, Yue Yiqin, among others, were lost; six months into the war, the Chinese Air Force inventory of frontline American Hawk IIs and IIIs and P-26Cs, and various others, were mostly superseded by faster and better armed Polikarpov I-15s and I-16s provided under the Sino-Soviet Treaty of 1937, including Soviet volunteer combat aviators. However, with the Fall of Nanking, the Fall of Wuhan, and the losses at the Battle of South Guangxi, the primary supply-line of China's "war of resistance" against the Imperial Japanese aggressions was dealt a further blow, not least of which the high-octane aviation fuel that was blocked-off from Chinese access, but still accessible to Imperial Japan. After suffering heavy losses in the Battle of Wuhan in October 1938, most active air force units were withdrawn hinterland for the defense of Sichuan against the most brutal terror-bombing raids ever inflicted upon civilian populations up to that time; the massive Japanese carpet-bombing campaigns, against targets in Sichuan, under the codenames Operation 100 and 101, were also fiercely contested solely by the Chinese Air Force units and anti-aircraft artillery units, sometimes with significant Japanese losses. But as China was not an aviation-industrial power at the time, losses continued to mount, while the Japanese forces enjoyed a highly developed aviation industry that saw constantly improving cutting-edge technological advancements giving Japanese aircraft distinct performance advantages in speed, agility, altitude/climbing rate and firepower that greatly placed the increasingly underpowered and underarmed Chinese fighter aircraft hampered with low-grade fuel at tremendous disadvantage. With the introduction of Mitsubishi A6M "Zero", the most advanced fighter aircraft of the time, the Imperial Japanese gained practically complete air-supremacy by 1941, giving the Japanese military high-command full confidence in proceeding with the implementation of Operation Z (the planning and rehearsals for the attack on Pearl Harbor). With the Japanese invasion of French Indochina, the United States enacted an oil and steel embargo against Japan and also the Lend-Lease Act on 11 March 1941, of which China was included as beneficiary on the 15th of March; the Republic of China government placed a request 1,000 aircraft to fight-back the Japanese. Through attrition and loss of their most experienced fighter pilots in the first half of the Second Sino-Japanese War, the Republic of China Air Force suffered much irreversible losses in combat against the Japanese, and as the US imposed and by the