China and Japan both claim the same tiny string of islands known as the Diaoyu islands to the Chinese and the Senkaku Islands to the Japanese.
Japan established an air defense identification zone over the islands in 1968 think of it as an extra buffer extending Beyond Japan's normal territorial airspace
Then last month China declared its own air defense zone one that overlaps Japan's Zone above the disputed Islands.
Under China's rules any aircraft entering the Zone must notify Beijing under threat of military force.