The treaty of San Francisco is not the correct document to refer to when it comes to Taiwan and China relations. Japan and China signed a separate peace. Essentially both are the same thing.
The part u would be referring to is that Japan renounces all claim to Formosa?
that would be article 2.
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Article 2 (Treaty between Japan and China)
It is recognised that under Article 2 of the Treaty of Peace which Japan signed at the city of San Francisco on 8 September 1951 (hereinafter referred to as the San Francisco Treaty), Japan has renounced all right, title, and claim to Taiwan (Formosa) and Penghu (the Pescadores) as well as the Spratley Islands and the Paracel Islands.
Article 2 (San Francisco)
(a) Japan recognizing the independence of Korea, renounces all right, title and claim to Korea, including the islands of Quelpart, Port Hamilton and Dagelet.
(b) Japan renounces all right, title and claim to Formosa and the Pescadores.
(appended)
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Now that leaves Formosa in limbo right? Since technically they never state that Taiwan now belongs to China. Notice however they do declare that Corea is an independant nation, but not formosa.
However for more damning legalities we refer to 2 previous documents.
1. The Cairo conference held between 2 allied greatpowers and Chinain 1943
2. The Instrument of Surrender of Japan in 1945 signed by Japan and most of the allies, including China.
In Cairo,
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Cairo Conference 1943
November, 1943
Released December 1, 1943
The several military missions have agreed upon future military operations against Japan. The Three Great Allies expressed their resolve to bring unrelenting pressure against their brutal enemies by sea, land, and air. This pressure is already mounting.
The Three Great Allies are fighting this war to restrain and punish the aggression of Japan. They covet no gain for themselves and have no thought of territorial expansion.
It is their purpose that Japan shall be stripped of all the islands in the Pacific which she has seized or occupied since the beginning of the first World War in 1914, and that all the territories Japan has stolen form the Chinese, such as Manchuria,
Formosa, and the Pescadores, shall be restored to the Republic of China.
Japan will also be expelled from all other territories which she has taken by violence and greed. The aforesaid three great powers, mindful of the enslavement of the people of Korea, are determined that in due course Korea shall become free and independent.
With these objects in view the three Allies, in harmony with those of the United Nations at war with Japan, will continue to persevere in the serious and prolonged operations necessary to procure the unconditional surrender of Japan.
http://www.yale.edu/lawweb/avalon/wwii/cairo.htm
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The instrument of surrender states that Japan unconditionally accepts all conditions listed down in Potsdam, which proceeds from the 3 conferences held before it, Yalta, Tehran and finally Cairo.
Since we know Cairo Conference was never brought into disrepute, or changed we can reasonably assume, the allies on condition of victory returned FORMOSA to China. Legally formosa was Japan's territory in 1943 due to the 1895 signing of the Shimoseki Agreement (is that correct i forgot the name of that document). Since Japan unconditionally surrendered to the Allies, and the Allies had stated that on victory the territories that once belonged to China and were "stolen" by Japan including FORMOSA would be returned to China.
So legally China has every right to claim Taiwan as their own territory. On the fact that ROC was stated in name, this ROC was the name of China back then. They changed the name to PRC in 1949. KMT fled to Taiwan and renamed Taiwan to ROC. Its the same as if i call my home ROC, but clearly i am not the ROC referred to in the document.
NOW ethnically, Taiwan might not have been originally Han Chinese, but technically so isn't Yunnan or the southern provinces but all are recognized as Chinese provinces. So why the disparities?
The reason i pointed out the ethno-linguistic argument in my first reply(although linguistic each of china's dialect is varied enough to be considered a language of its own) was the tone of the first post was concerning ethnicity. LordofTheNippleRings had posted a very good article on how the aborigines are being actually treated by the "NATIVE" TAIWANESE who are actually Han Chinese.
SO legally, I point out China has every right to claim Taiwan due to the 2 documents, and ethnically i point out that it makes no sense whatsoever to let a minority aboriginal population be the judge of whether their nation is independant especially when they are maltreated by the "native taiwanese".