ZHUHAI, China—A year after the collapse of the Soviet Union, a cash-strapped Kremlin began selling China a chunk of its vast military arsenal, including the pride of the Russian air force, the Sukhoi-27 fighter jet.

For the next 15 years, Russia was China's biggest arms supplier, providing $20 billion to $30 billion of fighters, destroyers, submarines, tanks and missiles. It even sold Beijing a license to make the Su-27 fighter jet—with imported Russian parts.

Today,...