As for Taiwan, we don't even recognize their flag for fear of repercussions with China. Everyone on the planet knows we'll stand by and watch if China chooses to invade - militarily. Economic and other repercussions are hopefully enough reason for them to leave well enough alone.
Naturally we'll get involved in civil wars if it is in our national interest, hell we'll even start one in a peaceful country. None of that speaks to a moral or legal obligation.
Interesting thing that I've stumbled upon recently is that the island of Taiwan, not to be confused with the Republic of China, may exist in an even more bizarre form of legal limbo so far as the United States Government is concerned than most people realize. And that was deliberate on the part of the United States back in the 1950's.
https://en.wikipedia.org/wiki/1943_Cairo_DeclarationThe 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 from the Chinese, such as Manchuria, Formosa, and The Pescadores, shall be restored to the Republic of China. ...
Okay, that makes the case for the CPC that under the context of there being a civil war in China at the time which was simply silent during the war, the Communists can latch on to that, as they obviously control parts of Manchuria today, among some other points.
But then we move later in the war:
https://en.wikipedia.org/wiki/Potsdam_Declaration#TermsThe 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.
While they reference the Cairo Declaration, the Potsdam Declaration does not, on its own affirm anything going to the ROC beyond "yeah, what that other document said"
https://www.archives.gov/exhibits/featured-documents/japanese-surrender-documentWe, acting by command of and in behalf of the Emperor of Japan, the Japanese Government and the Japanese Imperial General Headquarters, hereby accept the provisions set forth in the declaration issued by the heads of the Governments of the United States, China, and Great Britain on 26 July 1945 at Potsdam, and subsequently adhered to by the Union of Soviet Socialist Republics, which four powers are hereafter referred to as the Allied Powers.
...
We hereby undertake for the Emperor, the Japanese Government and their successors to carry out the provisions of the Potsdam Declaration in good faith, and to issue whatever orders and take whatever actions may be required by the Supreme Commander for the Allied Powers or by any other designated representative of the Allied Powers for the purpose of giving effect to that Declaration.
And given that Potsdam references Cairo, the Instrument of surrender now provides the basis for the Communist Party and the ROC to claim they've been granted sovereignty over Taiwan. In keeping with the Cairo declaration's statement of "shall be restored to the Republic of China."
Bringing us "General Order Number 1"
https://www.mofa.go.jp/mofaj/files/000097066.pdfThe senior Japanese Commanders and all ground, sea, air and auxiliary forces within China, (excluding Manchuria). Formosa and French Indo-China North of 16 degrees North latitude, shall surrender to Generalissimo Chiang Kai-Shek.
Except using as a basis of a claim gets dicey for the Communist party because..
The senior Japanese Commanders and all ground, sea, air and auxiliary forces within Manchuria, Korea North of 38 degrees North latitude, Karafuto, and the Kurile Islands, shall surrender to the Commander-in-Chief of Soviet Forces in the Far East
Which then brings us to the Treaty of San Francisco, where neither the ROC or the People's Republic were invited to attend.
https://en.wikipedia.org/wiki/Treaty_of_San_FranciscoWiki makes this claim, but I have not yet found the relevant documents.
According to the treaty's travaux préparatoires, a consensus existed among the states present at the San Francisco Peace Conference that, while the legal status of the island of Taiwan is temporarily undetermined, it would be resolved at a later time in accordance with the principles of peaceful settlement of disputes and self-determination, ideas that had been enshrined in the UN Charter.
I did find the supporting document for this one from the wiki page though:
In 1955, U.S. Secretary of State John Foster Dulles, co-author of the San Francisco Peace Treaty, affirmed that the treaty ceded Taiwan to no one; that Japan "merely renounced sovereignty over Taiwan". Dulles said that America "cannot, therefore, admit that the disposition of Taiwan is merely an internal problem of China."
But we'll get back to him.
https://treaties.un.org/doc/Publication/UNTS/Volume%20136/volume-136-I-1832-English.pdfArticle 2
(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.
...But with this treaty being signed in the fall of 1951, the ROK has retreated from the mainland of China and is holding out on the Island of Formosa(Taiwan). The United States is also involved in the Korean War at this time, and fighting against the Chinese PLA there. So it seems the US State Department carefully crafted that passage to be as vague as they possibly could. Japan ceded Formosa and Pescadore Islands in compliance with the prior declarations and agreements.. But it didn't specify who they ceded them to. But as the United States was the "Supreme Commander for the Allied Powers" operating in the Pacific Theater, that would mean Formosa technically defaults to the United States until
it decides who the sovereign is.
Now back to the US Secretary of State talking about Taiwan in 1955.
https://search.library.wisc.edu/digital/AK7LYDN2XR55Z78Q/pages/APJFBMLMVFOD2S8KThe Secretary said that we had in mind talking about the possibility of avoiding armed clashes. We certainly would not talk about the disposal of Taiwan. What we want is to assure that the problems can be worked out peacefully. As for the substantive aspects of the problems we must await the evolution of time, for if substantive matters are forced, no decision can be reached under present conditions and an armed clash would surely result. The CPR wants to get Taiwan which they haven't had for 60 years. Even the juridical position of Taiwan is in doubt. The United States also has an interest in Taiwan which we got away from Japan. Japan has merely renounced sovereignty over Taiwan which has not been disposed of by the peace treaty and not ceded to anyone. Consequently the United States also could assert a legal claim until Taiwan is disposed of by some means. We cannot, therefore, admit that the disposition of Taiwan is merely an internal problem.
But later that same year, we do get:
https://en.wikipedia.org/wiki/Sino-American_Mutual_Defense_TreatyARTICLE VI
For the purposes of Articles II and V, the terms "territorial" and "territories" shall mean in respect of the Republic of China, Taiwan and the Pescadores; and in respect of the United States of America, the island territories in the West Pacific under its jurisdiction. The provisions of Articles II and V will be applicable to such other territories as may be determined by mutual agreement.
Which is as close as the US comes to saying the ROC is the sovereign of Taiwan... Except Taiwan controlled more territory than that in 1955, and still do to this day.
The Senate's commentary on the Treaty as they ratified it is also "interesting" as well as per wiki:
"It is the view of the committee that the coming into force of the present treaty will not modify or affect the existing legal status of Formosa and the Pescadores."
And from there we get into the legal mouthful that Congress created with the Taiwan Relations Act after Jimmy Carter withdrew from the mutual defense pact.
The President- having terminated governmental relations between the United States and the governing authorities on Taiwan recognized by the United States as the Republic of China
...
For all purposes, including actions in any court in the United States, the Congress approves the continuation in force of all treaties and other international agreements, including multilateral conventions, entered into by the United States and the governing authorities on Taiwan recognized by the United States as the Republic of China prior to January 1, 1979, and in force between them on December 31, 1978, unless and until terminated in accordance with law.
...
the term “Taiwan” includes, as the context may require, the islands of Taiwan and the Pescadores, the people on those islands, corporations and other entities and associations created or organized under the laws applied on those islands, and the governing authorities on Taiwan recognized by the United States as the Republic of China prior to January 1, 1979, and any successor governing authorities (including political subdivisions, agencies, and instrumentalities thereof)
And once again, remember, Taiwan does control more than just the area they're describing, but the area they are describing are the areas that were part of Japan between the years of 1895 and 1945(or arguably 1952 when the San Francisco Treaty entered into force)
So it could be argued that the United States strategy of strategic ambiguity runs a little deeper than some would think. They're actually asserting that as Supreme Commander of the Allied Forces (Pacific) they are in fact the Sovereigns of Formosa and the Pescadores until such time that the ROC and PRC reconcile and unify in a peaceful manner. The ROC is simply acting as an administrator of the territory in the interim.

And until the United States makes that determination, Formosa and the Pescadores are not currently "a part of China" but rather "the former territorial possessions of Imperial Japan yet to be disposed of by treaty."
Which makes this read out in a more hilarious manner:
https://www.reaganlibrary.gov/archives/speech/united-states-china-joint-communique-united-states-arms-sales-taiwanIn the Joint Communique on the Establishment of Diplomatic Relations on January 1, 1979, issued by the Government of the United States of America and the Government of the People's Republic of China, the United States of America recognized the Government of the People's Republic of China as the sole legal government of China, and it acknowledged the Chinese position that there is but one China and Taiwan is part of China. Within that context, the two sides agreed that the people of the United States would continue to maintain cultural, commercial, and other unofficial relations with the people of Taiwan. On this basis, relations between the United States and China were normalized.
Where people need to remember that acknowledgement of a position is not the same thing as agreement with a position.