beginning of 1942 the ROCAF was practically annihilated by Japanese aircraft, particularly due to the introduction of the A6M Zero-fighter. The ROCAF was eventually supplemented with the establishment of the American Volunteer Group (known as the "Flying Tigers") with fast and heavily armed and armored Curtiss P-40 Warhawks, employing dissimilar hit and run tactics in defense of the primary wartime Allied supply-line into China known as "The Hump", between Kunming in Yunnan province and British bases in India, while the remains of the Chinese Air Force modernized and rebuilt its strength each year following Imperial Japan's attack on Pearl Harbor, with new commitment and support from the United States. The ROC Air Force was reconstituted into seven Groups, one separate Squadron and four Volunteer Groups. In 1939, after the USSR concluded a non-aggression pact with Germany, the Soviet Volunteer Group was withdrawn. By the end of 1941, the air force had 364 operational aircraft. Up to 100 of these were P-40Bs operated by the American Volunteer Group. U.S. replacement aircraft began to arrive in March 1942. They included A-29s, P-40s, P-43s, In 1944, the USAAF Fourteenth Air Force commenced joint operations in the China theatre. By this time the Chinese Air Force was mostly equipped with current operational aircraft types and was superior in all respects to the opposing Japanese air forces which remained. Units of the Chinese Airforce 1937–1945 1st Group, (bombers) Tupolev ANT 40 SB III, VI, North American B-25 Mitchell, Northrop Gamma 2E, Northrop A-17 2nd Group, (bombers) Tupolev ANT 40 SB III, Northrop Gamma 2E, Northrop A-17, 19 Lockheed A-29, 8 Avro 627 China built domestically as scout-bombers at Guangxi 30th Squadron: 20 Fiat BR.3 6th Group, (light bombers/scout-bombers) 21 Vought O3U/V-92C, Douglas O-2MC 7th Group, (light bombers/scout-bombers) 42 O2U Corsair, Douglas O-2MC 8th Group, (bombers) 2 Northrop Alpha 4 (converted locally to scout-bombers), 30 Ilyushin DB-3 (later into the Composite Group), North American B-25 Mitchell 10th Squadron 10 Savoia-Marchetti S.72, 6 Caproni Ca.111 19th Squadron 6 Heinkel He 111, 6 Tupolev TB-3 30th Squadron 6 Martin B-10 12th Group, (bombers) 10th Squadron, (bombers) 4 Vultee A-19 3rd Group, (fighters) Fiat CR.30, 9 Avro 626, Polikarpov E.15bis, E.15ter (E.15III, E.153), and E.16, Curtiss P-40C, P-51D, P-36 Hawk, 36 Gloster Gladiator Mk-1, P-66 Vanguard 8th Squadron (fighters 9 Fiat CR.32 17th Squadron (fighters) 11 Boeing Type 281 28th Squadron (fighters) Gloster Gladiator Mk-1 29th Squadron (fighters) Gloster