After suffering heavy losses in the Battle of Wuhan in October 1938, most active air force units were withdrawn hinterland for the defense of Sichuan against the most brutal terror-bombing raids ever inflicted upon civilian populations up to that time; the massive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).