You can look up the various legal arguments China uses to assert their claims, but this is a fairly impartial analysis of positions from all parties [0]. Significantly, it includes reactions from various international observers around the time, uninfluenced by current geopolitical tensions. Security council countries like Britain, USSR who outright recognize Chinese sovereignty (ROC or PRC) over Paracel, Spratly France who dropped their claims after French document supported Chinese claims, US who remained neutral (yup, US neutral). Other countries that supported Chinese claims: Germany, Japan herself:

>The Japanese accepted the terms of surrender set by the Potsdam Declaration, including the terms of the 1943 Cairo Declaration, which provided that Japan would return all Chinese territories that it had seized from China. Since Nansha and Xisha were occupied by Japan during World War II, they were returned to China. Thus, both the 1951 San Francisco Japan Peace Treaty and the 1952 Sino-Japan Treaty stipulated that Japan renounce all title and claim to Taiwan (Formosa), Penghu (Pescadores), the Parcel Islands and the Spratly Islands. Since then, Japan has not expressed any new position on China's claim of sovereignty over the islands in the South China Sea.

Claimants who recognize Chinese claims: Indonesia, North vietnam

Claimants who didn't: South Vietnam, Malaysia, PHILIPPINES

Philippines emphasized since what you found/quoted is basically the Philippine legal position from by their chief justice, Antonio Carpio from last month. Obviously every country have their own biases, arguments for nullifying others claims and again they are _legitimate_ arguments. But the more detached analysis and literature, like this analysis from the 90s before everything popped off, gives greater credence Chinese position. Just to pick on Philippines claims on Scarborough Shoal, based off terra nullius, from the mid 50s, i.e. Filipino claims to be first state/peoples to occupy the shoal, which is... well fantastically absurd since ROC claimed the shoals in early 1900s, which predates Philippines as a sovereign state. They're so late to the party that their argument is... China (and Vietnamese) claims that predates even San Francisco treaty just don't exist. Although this line is interesting, I can't find info on it anywhere.

> USSR’s motion to award the Spratlys and the Paracels to China was overwhelmingly rejected by the Peace Conference by a vote of 64 to 3

From context, speculating USSR probably wanted to award it to PRC... which doesn't invalidate the notion that ROC claims were recognized, as reflected by international reactions at the time, by who would eventually represent China, which later passed to PRC after in 71. Anyway, the Filipino position is stupid, legitimate, but stupid. Just like some of the new Chinese straight baseline arguments in response to PCA ruling. Look it up. Insert Arrested Development meme on the absurdity of maritime law. At the end of the day, it doesn't matter, each party is motivated to find minutiae to argue their case, geopolitical interests will shift positions (lol at US neutrality). Ultimately, IR is more rule by law than rule of law. Everyone has legal arguments, some older and better than others. Everyone also has a navy, some bigger and more modern than others. SCS dispute is going to be solved by 9/10th possessions and naval tonnage. There's too much on the line for any other way.

Edit, to add, since I just connected Carpio dot to point about Sino-Philippine energy exploration I made in another comment: In Philippines case, Carpios letter was followed up by Philippine declaring they would start oil exploration in SCS. The timeline suggesting Philippines asserting itself... but later revealed to be with Chinese blessing [1]. That's SCS dispute politics.

[0] https://scholarlycommons.law.case.edu/cgi/viewcontent.cgi?ar...

[1] https://amti.csis.org/a-philippine-china-deal-on-joint-devel...