The Japanese air raids against Chongqing had become increasingly intense and destructive as the "joint air strike force" of Imperial Japanese Army and Navy bomber formations began to comprise upwards of 150-200 bombers per raid, under the new codename ""Operation 101"" ("101"号作战), and massed defensive machine gun and cannon fire from the bomber formations were very effective against the slow Chinese fighter aircraft further handicapped with burning low-grade fuel, 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.