「臺灣主權未定論」，是美國政府於1950年6月，針對臺灣主權歸屬所提出的法律見解。主要的論述依據是，中華民國政府雖在二次大戰結束後根據《開羅宣言》接收臺灣，可是《開羅宣言》在法律上並不構成已將臺灣主權由日本之手移轉到中國之要件；臺灣主權的確定，還需要經由宗主國的放棄，以及接收國的主權繼承此一法律程序才算完備。換言之，「臺灣主權未定論」之所以會和「對日媾和」掛鉤，就是因為臺灣主權的移轉還未經舊宗主國放棄、新主權國繼承的法定程序，而美國可置喙「臺灣主權」問題，自然是因為美國負責主持「對日媾和」案，有決定日本領土範圍之權限。 　　 企圖將臺灣交由聯合國接管，是美國在韓戰爆發的第一時間內設計的方案，目的是防止臺灣落入中共之手。美國擬採用的方法，就是藉主導對日和約之便，首先讓日本在與聯合國會員國簽訂的和平條約中，放棄對臺灣的主權；其次，再將臺灣主權問題交由中、英、美、蘇四國共商解決，若四國在一年之內無法順利解決臺灣問題，則將問題交由聯合國處理。 所謂四國協商共謀解決臺灣問題，是二次大戰末期盟軍對日的決策模式。美國主張將臺灣問題先交四國協商，自然是代表對此一體制的尊重。可是，在韓戰爆發、東西對決態勢已明的1950年代，四國協商體制早已蕩然無存。美國之所以要重提四國協商，就是看準四國協商無法有效解決任何爭議，卻可為下一階段的美國自主外交政策鋪路。將臺灣交由聯合國託管，顯然才是美國的目的。至於如何處置蔣介石的政府，美國政府還沒有明確結論。 　　 不過，中共的介入韓戰，打亂了美國在遠東政策的原有佈局。為了牽制中共在朝鮮半島的軍事介入，動用一切可資利用的反共力量，就成為美國遠東政策的新指標。為了迅速動員一切可資利用的反共力量，重新支持蔣介石政府，也就成為美國新遠東政策下的一環。為了鞏固蔣介石政府，新的對日媾和草案中，撤回將臺灣問題先交由四國協商再交聯合國公議之提案。同時也停擺所有要將臺灣事務交由聯合國處理的部署。換言之，美國放棄將臺灣交由聯合國託管的既定政策，反而以扶植蔣介石政府為己任。在此一首要戰略的考量下，美國政府開始積極協助中華民國政府參與對日和約，以便中華民國政府在臺灣主權移轉的法定過程中，取得新繼承者的法人地位。 　　 1951年9月的舊金山和會，中華民國政府因其他與會者的強烈反對，不得參與。不過，卻得於翌年4月《舊金山和約》生效前，與日本政府簽訂與《舊金山和約》同等內容的《中日和平條約》。至於臺灣主權歸屬的問題上，由於《舊金山和約》只陳述了日本放棄對臺灣的管轄權，並沒有明文交待此等管轄權將移交給哪一國，是以《中日和約》同樣地對主權移轉的對象沒有明文交待。 　　 《舊金山和約》之所以不明文規定臺灣主權的歸屬，是因為中國沒有參與和會，而且在誰可代表中國仍有爭議的情況下，主導和會的英美兩國所刻意迴避的結果。至於《中日和約》蕭規曹隨，援用《舊金山和約》的條文，一則是美國不願因《中日和約》而破壞《舊金山和約》所樹立的規範，特別是《舊金山和約》原本就是英、美兩國妥協的產物，而英國一直主張臺灣主權應明文劃歸給中共的情況之下，得罪英國將會使舊金山的和約體制全面崩潰。二則是美國為了防備中共武力犯臺，仍需利用「臺灣主權未定論」做為擋箭牌。三則是中華民國為了爭取平等參與對日本的媾和會議，也希望《中日和約》是《舊金山和約》體制的一環，對臺灣主權歸屬不得明文宣示，願意委曲求全。 　　 不過，《中日和約》的條文中，雖沒有主權轉移的明確字眼，但有相關條文足以說明臺灣主權已轉移給中華民國之既有事實。《中日和約》第10條規定：「臺、澎人民及法人之為中華民國人民及法人」，以及議定書第2項規定：「中華民國之船舶應認為包括依照中華民國在臺灣及澎湖所已施行或將來可能施行之法律規章所登記之一切船舶；中華民國之產品應認為包括發源於臺灣及澎湖之一切產品」等條文規定，在在為中華民國已對臺灣享有管轄權之事實提供見證。換言之，《中日和約》雖然沒有將臺灣主權從日本手中轉到中華民國之事實明文交待，但用二段語文的方式，將移轉之事實宣告世人。前一段是日本宣布放棄對臺灣主權，後一段是日本承認中華民國對臺灣享有主權，前後兩段互為因果關係，不能因兩段之間的表述沒有轉移符號，而否定雙方的關聯。 　　 再者，日本政府承認中華民國享有對臺灣主權之立場，自1952年簽訂和平條約起就沒有改變。即使1972年日本與中共建交，簽訂《中日兩國政府聯合聲明》時，並沒有同意中共的主張，承認中共對臺灣有管轄權，反而強調日本政府堅守《波茨坦公告》第8條的立場，也就是，臺灣應歸還中華民國。此外，在日本的法院民事訴訟中，也一再表達了日本司法界認為自1952年《中日和約》生效日起，臺灣主權歸中華民國所有的基本認知。 　　 至於美國政府，雖然在《中日和約》簽訂後仍未放棄「臺灣主權未定論」的論述，可是也認為在《舊金山和約》簽訂、日本恢復法人地位後，國際社會中有權決定臺灣主權歸屬問題者只有日本與中華民國。美國的論述，只代表本國的立場，對臺灣主權的最終歸屬不具法律拘束力。美國支持中華民國對臺灣享有主權，但為防備中共的武力犯臺，故仍利用「臺灣主權未定論」為擋箭牌，替美國日後的軍事護臺行動預留活動空間。是以，在其他國家的意見皆不具法定程序拘束力的前提下，原本擁有所有權的日本，既然一再表態臺灣的主權已經由《中日和約》的簽訂，正式轉移到中華民國政府手中，臺灣主權移轉的合法性，自然不容質疑。

The notion of “undetermined Taiwan sovereignty” represented the legal view taken by the US government in June of 1950 in regard to the legal status of Taiwan. The crux of the issue lay in the Cairo Declaration, which, although holding that Taiwan would be returned to the Republic of China (ROC) at the end of World War II, did not create a legal procedure for passing Taiwan’s sovereignty from Japan to the ROC. The determination of Taiwan’s sovereignty could not be completed without a legal procedure involving Japan’s formal relinquishment of sovereignty and its acceptance by a successor state. Thus the question of “undetermined Taiwan sovereignty” became linked to the later peace treaty with Japan. The United States became involved in the issue of Taiwan’s sovereignty since it was responsible for the peace treaty and deciding what territories Japan would be allowed to retain. 　　Initially, Taiwan was to be governed by the United Nations, but when the Korean War broke out, the US quickly decided to prevent Taiwan from falling into the hands of the People’s Republic of China. The US accordingly used the negotiations over the peace treaty with Japan to have Japan renounce any rights to Taiwan. Then, the ROC, Britain, the US, and the Soviet Union would together decide the status of Taiwan. If these four nations could not resolve the issue in a year, then the question of Taiwan would be turned over to the UN. 　　This procedure rested on the alliance of the four nations that had cooperated to form the Allied policy toward Japan in the late stages of World War II. By advocating that the Taiwan issue should be decided by negotiation, the US followed the earlier model of Allied cooperation. In fact, however, the alliance system that supported such negotiations collapsed when the Korean War broke out and the Cold War began in the 1950s. Apparent American support for negotiations that were clearly impossible was in fact a step toward America creating a unilateral foreign policy. Yet the US obviously sought to make Taiwan a UN trustee. At the time, the US government still had no clear policy on how to deal with the government of Chiang Kai-shek. 　　However, the PRC’s involvement in the Korean War upset US Asian policy, leading the US to support the government of Chiang Kai-shek in Taiwan. To strengthen Chiang’s government, the US abandoned its original proposals as it pursued a peace treaty with Japan. As a key part of its new strategy, the US thus supported the ROC’s efforts to sign a bilateral peace treaty with Japan. This allowed the ROC to follow formal legal procedures in acquiring the legal status to succeed to the sovereignty of Taiwan. 　　The San Francisco Peace Treaty was signed in September of 1951 without the participation of the ROC government due to strong opposition from other countries. But the ROC soon signed a bilateral peace treaty with Japan in April 1952, and this Sino-Japanese Peace Treaty was essentially the same as the San Francisco Peace Treaty. Both treaties specified that Japan relinquished sovereignty over Taiwan, but neither specified who succeeded to legal jurisdiction over Taiwan. 　　The San Francisco Peace Treaty did not specify the sovereignty of Taiwan, since the PRC had not been invited to participate and the issue of what government represented China remained unsettled. Indeed, the Sino-Japanese Peace Treaty represented a compromise between the US and Britain. Britain insisted that the PRC should take over Taiwan. To avoid this controversy causing the collapse of the entire peace negotiations, the Sino-Japanese Peace Treaty simply followed the San Francisco Peace Treaty. Moreover, the US used the notion of “undetermined Taiwan sovereignty” in order to stop the Communists from invading Taiwan. The ROC accepted this compromise for the purpose of participating in the peace treaty with Japan. 　　Even without the sovereignty issue settled, the peace treaty allowed the ROC government to implement effective administrative control over Taiwan. Article 10 of the treaty stated that the Taiwanese people and the juridical person should be the people and the juridical person of the ROC. Item 2 of the protocol stated that the ships of the ROC should include those registered in Taiwan and that the manufactures of the ROC should include those regulated in Taiwan. The above clauses demonstrate the recognition of de facto administrative control of Taiwan. In other words, the Sino-Japanese Peace Treaty specified, first, that Japan relinquished its rights to Taiwan, and, second, that Japan recognized that the ROC succeeded to sovereignty over Taiwan. 　　Moreover, Japanese recognition of ROC sovereignty over Taiwan has remained unchanged since the peace treaty of 1952. Even the Joint Communique, signed in 1972 between the PRC and Japan, failed to recognize the PRC’s claims to Taiwan and indeed maintained the relevance of article 8 of the Potsdam Proclamation that stated Taiwan should be returned to the “Republic of China.” As well, the Japanese courts have maintained that the sovereignty of Taiwan belonged to the ROC since the Sino-Japanese Peace Treaty was signed in 1952. 　　As for the US government, although it did not abandon its view of “undetermined Taiwan sovereignty” with the signing of the Sino-Japanese peace treaty, since Japan regained its juridical status with the signing of the San Francisco Treaty, in international society only the Japanese and the ROC governments had right to decide the sovereignty of Taiwan. American support for ROC sovereignty in Taiwan stemmed from its strategic desire to prevent the Communists from invading Taiwan. “Undetermined Taiwan sovereignty” thus became a means of reserving scope for military action. However, Japan, which held the original sovereignty, has consistently regarded Taiwan sovereignty as having passed to the ROC government through formal legal procedure with the Sino-Japanese peace treaty. The legitimacy of the transfer of Taiwanese sovereignty is thus beyond doubt.