Taiwan is not an independent country. The historical and legal background is as follows.
Taiwan had been ceded to Japan in 1895. Following the 1895 Treaty of Shimonoseki, Japan exercised sovereignty over Taiwan and held title to its territory.
After Pearl Harbor, the US Congress declared war against Japan on Dec. 8, 1941. All military attacks against the four main Japanese islands and Japanese Taiwan were conducted by United States military forces, so it can be held that the United States is the "conqueror" of Japan and her overseas territories. In other words, the United States has "acquired" these areas under the principle of conquest, and the disposition thereof must be conducted according to the laws of war.
General Order No. 1 was issued on Sept. 2, 1945. President Harry Truman approved this Order before its promulgation, the United States is the "conqueror" of Japan and her overseas territories, and General MacArthur is the head of the United States military forces. Hence the strongest presumption would be that United States is the principal occupying power. Importantly, Article 23 of the San Francisco Peace Treaty (SFPT) fully confirms this.
Although the October 25, 1945, Japanese surrender ceremonies in Taiwan were ostensibly conducted on behalf of the Allies, the ensuing military occupation of Taiwan is being conducted on behalf of the "principal occupying power," which is the United States. Military occupation is conducted under military government. Under the SFPT, Republic of China (ROC) troops in Taiwan are considered a subordinate occupying power. This is "agency." Oct. 25, 1945 marks the beginning of the military occupation of Taiwan. USMG is the principal. The ROC troops are the agent.
There has been no change in this status to date. Taiwan remains as occupied territory of the United States of America. The ROC on Taiwan is (1) a subordinate occupying power, beginning Oct. 25, 1945, and (2) a government in exile, beginning mid-December 1949. The ROC exercises effective territorial control over Taiwan, but not sovereignty.
On Oct. 25, 2004, Secretary of State Powell said: "Taiwan is not independent. It does not enjoy sovereignty as a nation."
Under United States laws, "Taiwan" is a sub-sovereign entity, and the nomenclature of "Republic of China" is not recognized after Jan. 1, 1979. According to the post-war peace treaties, there has been no transfer of the territorial sovereignty of Taiwan to either the Republic of China or the Taiwan governing authorities.
Also it is significant to note that Secretary Powell was a military man, and he understands military occupation. As explained above, Taiwan is occupied territory of the United States of America. Importantly, after the peace treaty cession from Japan there are numerous US constitutional rights associated with that status. .
The Taiwanese government officials often say that the Taiwanese people enjoy popular sovereignty, but in international law the notion of popular sovereignty mainly revolves around the fact that the local people have the right to elect their own representatives to the government.
If we examine the subject of territorial cession from the 1700's to the present, it is clear that territorial cession is an action between governments, and territorial sovereignty is owned by a government. In Secretary Powell's remarks, obviously he was speaking of "territorial sovereignty." The government in Taiwan does not have territorial sovereignty over the areas of Formosa and the Pescadores. In other words, the government in Taiwan does not have "territorial title" to the areas of Formosa and the Pescadores, and hence does not meet the criteria for "statehood" necessary to enter the United Nations.