J-7:
In 1961, as tensions between the USSR and the PRC reached a fever pitch, the Soviets transferred blueprints and materials associated with its new MiG-21 interceptor to China. The offering represented an effort to bridge part of the gap, and suggest to China that cooperation between the Communist giants remained possible.
The offering didn’t work. Sino-Soviet tensions continued to increase, nearly to the point of war in the late 1960s. The Chinese worked from the blueprints and other materials, and eventually produced the J-7, a virtual copy of the MiG-21. The Chinese eventually sold the J-7 (F-7 export variant) in direct competition with the MiGs sold by the Soviets. Indeed, after the US-PRC rapprochement of the early 1970s, the Chinese sold J-7s directly to the Americans, who used them as part of an aggressor squadron to train US pilots to fight the Soviets.