 Complete the following phrase:  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 Chinas continuing war of resistance against the Imperial Japans 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 