As the war between China and the Empire of Japan continued into its fourth year, material support from the U.S. had long since mostly disappeared, and reliance on mutual agreement to fight the Empire of Japan had been largely sustained through the Sino-Soviet Treaty in China's continuing war of resistance against the Imperial Japan's aggression and occupation. The Chinese Air Force had mostly converted to Soviet-made Polikarpov I-15 and I-16 series of fighter aircraft, and the losses at the Battle of Wuhan and the Battle of South Guangxi had pushed the Chinese hinterland to the new wartime capital in Chongqing, Sichuan province, and all seaports that previously supplied China's most important materials for the war effort, including quality aviation fuel needed for proper fighter aircraft performance, have become cut off.