American military strategy is premised on the small Daioyu islands (Senkaku for Japan) as being a part of Japan, but the situation is a lot more complicated.
In fact, Japan was new to even Okinawa in the 1890s, and this southwestern maritime expansion came rather late in its history. This region was traditionally part of the Ryukyu Kingdom and the islands often paid tribute to Chinese dynasties, which included the islands on their maps after 1534.
Japan annexed the islands after the First Sino-Japanese War in 1895, but then Japan lost them, China claims, in the Potsdam Declaration that ended WWII, where Japan was only entitled to its main five islands and minor islands as designated.
So it depends on one’s default assumption…
The Daioyu Islands became part of US occupied territory, like Okinawa, and all of this was handed back to Japan in 1972 with the Okinawa Reversion Treaty. But China insists that the islands should have been given back to China right after WWII and were not. Indeed, during the Cold War with the Soviet Union, it would have been unthinkable for the US to hand these islands over to a communist power – but the history of the islands should probably have led to that.
In other words, China has a solid claim on the Daioyu Islands. At the very least, American thinking should be looking for ways around a traditional conflict, perhaps even with a “joint sovereignty” solution, rather than act overly confident that it is on the right side of history with these islands. When it is not.