The only authoritative treaty on the matter is the WW2 Peace Treaty signed in San Francisco in 1951. I invite everybody to read it.This is wrong analogy. If the USSR advanced into European countries, it would be naked invasion against other sovereign countries and the invaded countries would fight them with all they had. The USSR did not have the will to launch such offensive. On the other hand, when Nazi Germany invaded the USSR, the Russian people made huge sacrifices to fight against the Nazi. I'm pretty sure you're very familiar with that part of the history.
The notion that the US's "credibility" would be on the line and would be "kicked out of Western Pacific" is ludicrous and is argued to deter China's inevitable unification of Taiwan. Weighting someone's credibility, even that of the almighty U.S.A., against Chinese territory and sovereignty is way out of proportion. That once China unifies Taiwan, the US would have to get out of Western Pacific (what does that even mean?) is weird logic. We're not talking about China invading Japan here, which is totally different from China recovering Taiwan.
So you're confusing Taiwan, a legitimate territory of China, with Europe or Japan, both of which are independent sovereign countries of their own.
As to importance of Taiwan to US national interests, here is a perspective from 1950:
Memorandum on Formosa, by General of the Army Douglas MacArthur, Commander in Chief, Far East, and Supreme Commander, Allied Powers, Japan
[Tokyo,] 14 June 1950.
[... omitted first 7 chapters]
Formosa represents a political area of no less importance to western ideology than other areas in the Orient. The Taiwanese are a homogeneous racial group who as individuals have resisted the intrusion of foreign blood. Although Formosa was promised to China as a consequence of World War II this promise was given in consonance with a political situation entirely different than that which now exists. There is every basis from a moral standpoint to offer to the Taiwanese an opportunity to develop their own political future in an atmosphere unfettered by the dictates of a Communist police state. In view of the moral implications, as well as the geographic proximity of this area to other endangered peoples on and near the periphery of China, the future status of Formosa can well be an important factor in determining the political alignment of those national groups who have or must soon make a choice between Communism and the West.
There can be no doubt but that the eventual fate of Formosa largely rests with the United States. Unless the United States’ political-military strategic position in the Far East is to be abandoned, it is obvious that the time must come in the foreseeable future when a line must be drawn beyond which Communist expansion will be stopped. As a means of regaining a proper United States posture in the Orient it is apparent to me that the United States should initiate measures to prevent the domination of Formosa by a Communist power. I am equally certain that it would be a fundamental error with regard to any part of the Orient to fail to take appropriate measures in those areas still open to our influence.
At this time I am unable to recommend the exact political, economic and military measures which should be taken to prevent the [Page 165]fall of Formosa either into the hands of a potential hostile power or into the hands of a power who will grant military utilization of Formosa to a hostile power. It is my firm conviction that a realistic estimate of requirements can only be based upon a physical survey of the area made by experienced military, economic and political observers. I concur whole-heartedly with the recommendations made by the Joint Chiefs of Staff on 23 December 1949 to the effect that the Commander-in-Chief Far East should make an immediate survey of the need and extent of the military assistance required in Formosa in order to hold Formosa against attack. Although this recommendation was apparently not acceptable at the time to the National Security Council, I note that the Joint Chiefs reaffirmed this recommendation on 4 May 1950.
Formosa has not yet fallen to Communist domination. There are conflicting reports as to the capability and will of the Chinese Nationalist Forces as now constituted and equipped to prevent either the military or political conquest of the island of Formosa. I cannot predict what the cost may be of preventing Communist domination of that island, although I have advised the Joint Chiefs of Staff what the cost may be if such an event transpires. I am satisfied, however, that the domination of Formosa by an unfriendly power would be a disaster of utmost importance to the United States, and I am convinced that time is of the essence. I strongly believe that the Commander-in-Chief Far East should be authorized and directed to initiate without delay a survey of the military, economic and political requirements to prevent the domination of Formosa by a Communist power and that the results of such a survey be analyzed and acted upon as a basis for United States national policy with respect to Formosa.