IMHO the lawsuit is a crank lawsuit that is simply a waste of the Court's time, which is sad because Dick Hartzell, a tireless campaigner, has done more for the foreign community here than any other foreigner who has lived on the island, and I admire his work very much (and wish he would focus his efforts on that!). But the simple fact of the matter is that the US position is that it has no sovereignty over Taiwan, and neither does anyone else. It is also a simple matter that the US position on sovereignty is a political question, not a legal one, to be decided by the legislative and executive branches of the government in accordance with the US Constitution (see Goldwater v. Carter, a famous case about US relations with Taiwan). No document produced subsequent to the San Francisco Peace Treaty by either branch recognizes US sovereignty over Taiwan. End of discussion.
Hartzell/Lin's arguments are laid out in a number of places. Readers could consult this long discussion at Forumosa, the number 1 expat forum in Taiwan, run by Anthony van Dyck, one of the nicest people I know. A quick version is online here. A short version is this Taipei Times piece from 5 years ago, in which Hartzell observes:
Step 3: In the post-war peace treaties, the sovereignty of Taiwan was not awarded to the ROC, hence Taiwan remains under the administrative authority of the US military government, and this is an interim status condition. In the San Francisco Peace Treaty, Article 4b clearly states that the US military government has final disposition rights over "Formosa and the Pescadores."Hartzell apparently believes that Taiwan belongs to the PRC -- or perhaps only that such a conclusion is a consequence of current US policy -- and the US is holding it in trust for Beijing. This is not a position that I as an ethical human can support. The chain of sovereignty is quite clear, and terminated in April of 1952 when Japanese sovereignty was effectively ended by the San Francisco Peace Treaty.
In addition, Article 23 reconfirms the US as the principal occupying power.
In effect, the US is holding the sovereignty of Taiwan "in trust," and in the Shanghai Communique the US president is making arrangements for the future handover of this sovereignty to the People's Republic of China, which is recognized as the sole legitimate government of China! However, at the present time, Taiwan is still under US administrative authority, and should be enjoying "fundamental rights" under the US Constitution, as in all other US overseas territories.
As Lin and Hartzell correctly note in their presentation, the decision not to award sovereignty was deliberate, and remains the position of the US and other powers even today: no nation has sovereignty over Taiwan.
The island of Taiwan belongs to the people of Taiwan, and only to them.
UPDATE: Since I published this post, Hartzell and Lin have a new and more pro-TI position over on their Taiwan civil government website:
The Central Administrative Commission of the Taiwan Civil Government, in order to allow the public to more easily understand the nature of this Government's organization, has chosen the motto of "US Nation-Building Group." This motto clearly indicates that Taiwan at present is temporarily an overseas territory under the jurisdiction of the United States Military Government (USMG). In the future, after Taiwan's true international legal status is fully recognized, the Taiwanese people will proceed along the path of nation-building with the help of USMG.UPDATE 2: My good friend Echo Taiwan has a completely different view of this case.
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