se scholars point to this among other things to argue that Japan didn't have authority over the islands.
Koga was however given rights to cultivate the land of four of the islands in 1896 and is said to have done so till the 1920s. His son is said to have bought the islands soon after his death.
To build its case on sovereignty over the islands Japan also points to the 1895 Treaty of Shimonoseki, in which China agreed to cede Taiwan to Japan along with it all the islands that belonged to Taiwan. However, the treaty didn't mention the Diaoyu/Senkaku Islands by name which doesn't help clarify matters.
In fact Lohmeyer points out that the ownership of these islands in particular were sidestepped during the Cairo Declaration in 1943, decree 667 by the UN on the Japanese territory and so on. From Lohmeyer:
"As early as 1372 Chinese Imperial Envoys used the islands as a navigation point. China argues that even in the 18th century Japanese maps showed the islands to be Chinese territory. Taiwan’s claim to the islands concentrates on the argument that Taiwan had occupied them for a long time and that the islands form a natural prolongation of the Taiwanese continental shelf. Japan claims that until the end of the 19th century the islands were “terra nullius” [land belonging to no one].
Since 1894, the islands have been under Japanese administration. In the Sino-Japanese war in 1895, China was forced to cede Taiwan and all the islands belonging to her to Japan. Later, in the Sino-Japanese treaty of 1952 the parties signed that as a consequence of the war all prior agreements were hereby null and void. After World War II, the U.S. administered the disputed islands in conjunction with the Okinawa Islands. In 1972, the U.S. returned administration to Japan."
Second, after years of conflict, Sino-Japanese relations have continued to be strained in recent years.
China helped the allies fight Germany during World War I on the condition that certain territories held by Germany like the Shandong peninsula be returned to China. Instead they were handed over to Japan which China viewed as a betrayal.