There is a territorial dispute over a group of uninhabited islands called the Senkaku Islands in Japan and the Diaoyu Islands in China.
Aside from the 1945 to 1972 period of administration by the US as part of the Ryukyu Islands, the archipelago has been controlled by Japan since 1895.
China began to question the sovereignty of the islands in late 1970 when evidence of oil reserves were discovered. The territory is also close to key shipping lanes and rich fishing grounds.
The Chinese claim there is evidence to prove that before the First Sino-Japanese War, the territory belonged to China but was later seized by Japan. They argue that the islands should be returned to China as the rest of Imperial Japan's conquests were returned in 1945.
The islands are included within the Treaty of Mutual Cooperation and Security between the United States and Japan which means the US would be obliged to come to Japan's aid in the event of war.
Donald Trump said on Friday that he would 'walk' from Japan and other US allies if they don't commit to defending America and pay their fair share of NATO expenses.
"You know we have a treaty with Japan, where if Japan is attacked, we have to use the full force and might of the United States" he said in Iowa.
"But if we're attacked, Japan doesn't have to do anything, they can sit home and watch Sony television, OK?"