When was Japan' s present territory determined?

In 1945, when Japan was defeated in World War II, its land was reduced to its present size, the same size as it was prior to the Sino-Japanese War in 1894.
Today Japan consists of four main islands: Hokkaido, Honshu, Shikoku, and Kyushu, plus the Okinawa Islands, and thousands of other smaller islands.
Okinawa remained under American control even after the San Francisco Peace Treaty in 1951; it was returned to Japan in 1972.
Until the end of World War II, Japan owned Sakhalin, the Kuril Islands, Korea and Taiwan, which were regarded by the rest of the world as taken by aggression.

Currently, Japan claims the southern part of the Kuril Islands including the Habomai Islands, Shikotan, Kunashiri, and Etorofu as part of its territory, and negotiations with Russia are ongoing.

Another area in dispute is the Senkaku Islands, which are under the jurisdiction of Ishigaki City, 160 km north of the Yaeyama Islands in Okinawa Prefecture, for which not only Japan but also China and Taiwan claim territorial rights.