The annex towards the bottom of the page is what I wanted to link to. It is relevant to the China/Taiwan discussion. General MacArthur explains in 1950 the reasoning on why we had to intervene militarily in the Chinese civil war, protect the Chiang Kai Shek and the Kuomintang, create a safe haven for them in Taiwan (Formosa) and more importantly deny mainland China from the island of Taiwan.

So I included it part of it here (HN limits the length):

[Annex]

Memorandum on Formosa, by General of the Army Douglas MacArthur, Commander in Chief, Far East, and Supreme Commander, Allied Powers, Japan
top secret
[Tokyo,] 14 June 1950.
1.
Since the fall of 1948 when the military capability of the Chinese Communist to engulf all of the mainland of China became clearly evident I have been concerned as to the future status of Formosa and I have been convinced that the strategic interests of the United States will be in serious jeopardy if Formosa is allowed to be dominated by a power hostile to the United States.10 In my personal conversations with distinguished civilian and military representatives of the Government of the United States who have visited this Headquarters during the past eighteen months I have reiterated the premise that Formosa should not be allowed to fall into the hands of a potential hostile power or of a regime which would grant military utilization of Formosa to a power potentially hostile to the United States. On the 29th of May last I forwarded to the Joint Chiefs of Staff my estimate of the strategic consequences which would result from the capture of Formosa by the Chinese Communists.
2.
The front line of the Far East Command as well as the western strategic frontier of the United States rests today on the littoral islands [Page 162] extending from the Aleutians through the Philippine Archipelago. Geographically and strategically Formosa is an integral part of this offshore position which in the event of hostilities can exercise a decisive degree of control of military operations along the periphery of Eastern Asia. In the event of a war United States striking forces based on this line would have the capability to interdict the limited means of communication available to the Communists and deny or materially reduce the ability of the USSR to exploit the natural resources of East and Southeast Asia. This essential capability on the part of the United States is dependent to a large degree upon the retention of Formosa by a friendly or a neutral power.
3.
The geographic location of Formosa is such that in the hands of a power unfriendly to the United States it constitutes an enemy salient in the very center of that portion of our position now keyed to Japan, Okinawa, and the Philippines. At the present time there is on Formosa a concentration of operational air and naval bases which is greater than any similar concentration on the Asiatic mainland between the Yellow Sea and the Strait of Malacca. Additional bases can be developed in a relatively short time by an aggressive exploitation of World War II Japanese facilities not now utilized by the Chinese Nationalist Forces. Formosa bases are 100 miles closer to Okinawa than any point on the Chinese mainland and are 150 miles closer to Clark Field and Manila than any other area which could be acquired by Communist military forces. An enemy force utilizing those installations currently available on Formosa could increase by 100 percent the air effort which could be directed against Okinawa as compared to operations based in China proper and at the same time could direct damaging air attacks with fighter type aircraft against our installations in the Philippines which are currently beyond the range of fighters based on the mainland of Asia.
4.
As a result of its geographic location and base potential, utilization of Formosa by a military power hostile to the United States may either counterbalance or overshadow the strategic importance of the central and southern flank of the United States front line position. Formosa in the hands of the Communists can be compared to an unsinkable aircraft carrier and submarine tender ideally located to accomplish Soviet offensive strategy and at the same time checkmate counteroffensive operations by United States Forces based on Okinawa and the Philippines. This unsinkable carrier-tender has the capacity to operate from ten to twenty air groups of types ranging from jet fighters to B–29 type bombers as well as to provide forward operating facilities for the short-range coastal submarines which are predominant in the Russian Asiatic Navy. If Formosa should be acquired by the Chinese Communists and bases thereon made available [Page 163] to the USSR, Russia will have acquired an additional “fleet” which will have been obtained and can be maintained at an incomparably lower cost to the Soviets than could its equivalent of ten or twenty aircraft carriers with their supporting forces.
5.
Current estimates of Soviet air and submarine resources in the Far East agreed to by both Washington and Tokyo military intelligence agencies satisfy me that the Russians have the capability to extend their forces southward from their present positions and still maintain an imposing degree of military strength in both the Maritime Provinces and the Chinese seaboard. The ability of the USSR–Chinese Communist hordes to meet promptly logistic requirements either by improvisation or by the import of critical materials from Europe is being demonstrated daily by military activities extending from Tientsin to the southern border of China. The interest of the USSR in the southward displacement of termini of the Trans-Siberian Railroad has been reported by competent observers whose information indicated that rail lines are being extended through China southward from the vicinity of Lake Baikal and eastward from Russia Turkestan in the vicinity of Alma Ata. A trans-Siberian railhead in the vicinity of Shanghai would materially assist in the logistic build-up of Formosa. Pending the actual outbreak of hostilities United States military forces will be unable to prevent the stockpiling of essential military supplies on Formosa if that area is acquired by the Communists.
on 4 May 1950.
Douglas MacArthur