As an incident that shows how Japan was treated unfairly and in an evil way, let me explain about two islands which have territorial disputes.
1) Senkaku islands(釣魚臺列嶼)
There are islands called Senkaku islands in East China Sea, which we have a territorial dispute with China at present. Japan has an effective control over the islands. But Chinese ships or submarines which disguise as fishing boats invade into the territorial waters and activists land on the islands, which is controversial.
However, this problem is actually so simple that it will be possible to understand it by finding out these two facts.
1) Reserves of submarine oil field
This territorial dispute had not existed until 1971. However, after a great reserve of an oil field was discovered, China started to insist on their clam of the territory.
2) Who determines the borders?
Japan takes effective control over these islands. However, it was not decided by Japan. After the Pacific War, Japan surrendered its sovereignty to the allied nations which won the war. As a result, Japan lost a lot of lands, and could not even decide on its borders by itself. And Japan had to receive whatever the allied army told them to receive.
Then who decided on the borders? That was done naturally by the nations which won the II World War. Which were those nations? They are mainly 4 countries which stand for ABCD, that is, America, Britain, Chaina and Dutch.
These nations judged Japan as the victorious nations, dividing Japan and determining its borders. In short, China, one of the victorious nations, was part of determining the borders of Japan.
The fact that Japan takes control over Senkaku islands shows that Japan just administrates what is assigned by the victorious nations including China, whether Japan likes it or not. Therefore, it is obvious that these islands are part of the territory of Japan.
Even after the war was over, the U.S. ruled over Okinawa and the islands around it. In 1972, 27 years after the war was over, Okinawa was returned to Japan. Senkaku islands were also ruled by the U.S. with the borders determined by the victorious countries. And after that they were returned to Japan. Japan did not have any say about that.
Japan only rules over Senkaku islands which were returned by the allied nations.
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2) Takeshima island (Liancourt Rocks 독도)
The island called Takeshima is occupied by Korea illegally and Korea insists that it belongs to Korea.
This island belongs to Japan from olden times.
Also when the victorious nations decided on the borders again, this island belonged to Japan. Just before Japan became independent in 1952, this island got occupied by Korean first president Syngman Rhee and has been occupied since.
This is why it is illegal occupation in every way. Japan asks for a peaceful solution to be done through the international court, but Korea is avoiding it.