Some years ago, a very level-headed analysis of the effects of the 1943 Cairo Conference and 1945 Potsdam Conference was offered by an Assistant Professor at a university in Taiwan. He wrote:
The first sentence of the 1943 Cairo Statement, released after the Cairo Conference, reads:
The several military missions have agreed upon future military operation against Japan.
A few themes need to be analyzed concerning the conclusion of the Cairo Conference.
First, the full title of the conclusion of the Cairo Conference was "A Statement Regarding War Against Japan." No agreement or arrangement was signed between the three Allies (United States, China and United Kingdom). The only conclusion made was a "statement." Clearly, the Vienna Convention on the Law of Treaties does not attribute "statement" as an international agreement.(FN: 1)
Second, the conclusion of the Cairo Conference was "expressed" by the "military missions." It is clear that it was only an expression of intention, and in formal sense, is not a legal document.(FN: 2)
Third, the conclusion of the 1943 Cairo Conference was only termed as "declaration" in Article 8 of the 1945 Potsdam Proclamation. In other words, it was not termed as a declaration when issued 18 months earlier on Dec. 1, 1943, with no treaty effect. It is questionable that the Potsdam Proclamation of 1945 can accord the Cairo Statement of 1943 with the status of "declaration."
Even if the conclusion of the Cairo Conference has been entitled "declaration," according to the Law of Treaties, declaration usually denotes a treaty that declares existing law, with or without modification, or creates new law, after a conference of Heads of States.
If the now termed "Cairo Declaration" is to create new law, there must be rights and duties between the signatories. Reviewing the Cairo Statement, it revealed (1) the Allies’ determination to bring unrelenting pressure against Japan (2) the Allies’ unwillingness of territorial expansion (3) the Allies’ purpose to see Japan stripped of all the territories she has seized, occupied, or stolen, and (4) the Allies’ continuous fighting to procure the unconditional surrender of Japan.
It is especially worth noting that in this conference, "Taiwan’s restoration to China" was illustrated as "the Allies’ purpose." Neither duty nor rights among the three Allies were observed, nor were relations regulated. Thus, it would be wrong to accord the Cairo Statement with the status of a formal treaty.(FN: 3)
As for the Potsdam Proclamation, there are three documents, namely Potsdam Declaration Regarding Germany (June 5, 1945), Protocol of Proceedings approved at Berlin (Potsdam, August 2, 1945) and Potsdam Proclamation by Heads of Governments of the United States, United Kingdom and China (July 26, 1945). Only the third document related to the legal status of Taiwan. It reads:
The terms of the Cairo Declaration shall be carried out and Japanese sovereignty shall be limited to the islands of Honshu, Hokkaido, Kyushu, Shikoku and such minor islands as we determine.
As noted, this Potsdam Proclamation provided a title for the Cairo Statement, it was termed the "Cairo Declaration." The Potsdam Proclamation also vowed to carry out the Cairo Declaration. Obviously, if the conclusion of the Cairo Conference was a statement of intention, so was the Potsdam Proclamation.
It must be noted that during the period these two documents were issued, Japan was a legal sovereign of Taiwan based on the Shimonoseki Treaty. Japan then had acquired a de jure title over Taiwan and had not yet surrendered it to the Allied Powers, nor was Japan involved in the drafting or promulgation of the above-mentioned documents.
Therefore, it is fair to say that these two documents were only unilateral statements of intention that excluded Japan’s involvement, and their effectiveness or "legally binding power" was very much questionable.