It's not complicated. The Senkaku Islands were discovered, claimed and named by China a long long time go. China claims it was the 14th century but the exact date does not matter. What matters is Japan also knew the islands belonged to China before taking them during the first Sino Japan war.
After WW2 the U.S made the mistake of including the Senkaku islands as part of the whole Japanese Ryukyu Islands group. Unsurprisingly in the 70's when the U.S handed control of the Ryukyu Islands group back to Japan it also mistakenly included the Senkaku islands.
It could be argued that China took too long to claim the islands back for itself but considering nobody listened to anything China had to say from the 50's onward, its not a surprise. What matters is that Japan did know the Senkaku islands belonged to China but excepted them from the U.S anyway.
As time moves forward the islands are not inhabited and thus the rightful transfer ownership back to China will harm nobody. Personally I couldn't care less about the island group but if had to make a judgement id rule in favor of China on this one. The Japanese case for the islands is a bit weak.
