Although U.S. policy in early 1950 was not directly poised to prevent a Communist Chinese invasion of the Nationalist Chinese stronghold on Taiwan, the outbreak of the Korean War changed the equation.
On June 27, 1950, the United States Seventh Fleet sailed into the Taiwan Strait to prevent the Korean conflict from spreading south.
The appearance of the Seventh Fleet caused the Chinese Communists to transfer troops poised for an invasion of Taiwan to the northern border of Korea.
Source: Milestones: 1953–1960 – Office of the Historian
On 27 June 1950, President Truman issued the following statement:
The attack upon Korea makes it plain beyond all doubt that communism has passed beyond the use of subversion to conquer independent nations and will now use armed invasion and war. It has defied the orders of the Security Council of the United Nations issued to preserve international peace and security. In these circumstances the occupation of Formosa by Communist forces would be a direct threat to the security of the Pacific area and to United States forces performing their lawful and necessary functions in that area. Accordingly, I have ordered the 7th Fleet to prevent any attack on Formosa. As a corollary of this action, I am calling upon the Chinese Government on Formosa to cease all air and sea operations against the mainland. The 7th Fleet will see that this is done. The determination of the future status of Formosa must await the restoration of security in the Pacific, a peace settlement with Japan, or consideration by the United Nations.