I felt this fit best in r/askhistorians since this has to do with historical claims to territory:
Most countries in the Far East (China, Japan, Vietnam, the Philippines, etc.) have some sort of claim to a number of different islands and bodies of water in the South China Sea. What types of documents or historical events are they claiming that can actually be verified as legitimate? It doesn't seem like there are any significant indigenous peoples to these areas, and I don't recall ever hearing about Asian countries extensively colonizing islands outside their immediate coastal waters.
The Japanese claim to the Senkakus can be boiled down essentially to the following:
*Okinawa belongs to Japan; it was a protectorate and then formally annexed in 1878, and was returned to Japan in 1972. *The Senkakus have traditionally been administered as part of Okinawa. Like the rest of Okinawa, they were administered by the United States from 1945 until 1972 when they were handed back over to Japan as part of the reversion of Okinawa. *The islands were found to be terra nullius when surveyed in 1895 -- they were uninhabited and thus there's no way China could claim them. *The PRC and Taiwan didn't start claiming the Senkakus until after a UN report from 1969 suggested they might be on top of some oil and gas reserves.
Here is the official position of the Ministry of Foreign Affairs.
Japan does not to the best of my knowledge claim anything in the South China Seas. In fact, the terms of the Treaty of San Francisco require them to renounce any territorial claims outside the four main islands and the islands administered by the US after the war.
Daniel J. Dzurek, American academic geographer and former Chief of the Spatial, Environmental and Boundary Analysis Division of the United States' Department of State has a book "The Spratly Islands Dispute: Who's on First?".