 Six months into the war, which is considered the beginning of World War II in Asia, the Chinese Air Force inventory of frontline American Hawk IIs and IIIss and P-26Cs were superseded by faster and better armed Polikarpov I-15s and I-16s as support from the Soviet Union grew and American support faded.Through attrition and loss of their most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 ultimately suffered irreversible losses in combat against the Japanese, and by the beginning of 1942 the ROCAF was practically annihilated by Japanese aircraft, particularly with the introduction of the Mitsubishi A6M Zero.The ROCAF was eventually supplemented with the establishment of the American Volunteer Group (known as the "Flying Tigers") with heavily armed and armored Curtiss P-40 Warhawks, and subsequently rebuilt each year following Imperial Japan's attack on Pearl Harbor with new aid and vigor from the United States.