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 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.