 Complete the following phrase:  It was a war of attrition for the Chinese pilots, as many of their most experienced ace fighter pilots, such as Lieutet , Colonel Kao Chih-hang, Yue Yiqin, among others, were lost; six months into the war, the Chinese Air Force inventory of frontline American Hawk IIs and IIIs and P-26Cs, and various others, were mostly superseded by faster and better armed Polikarpov I-15s and I-16s provided under the Sino-Soviet Treaty of 1937, including Soviet volunteer combat aviators. However, with the 