Even though the isolation and containment of mainland China had been a standing principle of US foreign policy, President Nixon, with his strong anti-Communist background, felt that it was time to adopt a more conciliatory attitude.
According to Hsu in The Rise of Modern China
The Nixon-Kissinger world perspective was a product of Realpolitik. It postulated that the bipolarization of the post-World War II era, characterized by American and Soviet domination had drawn to a close. In the next decade, and probably the rest of this century, there would exist five power centers in the world: the United States, the Soviet Union, China, Japan, and Western Europe. Of the five, only China was isolated from the world community and therefore it was “imperative” that it be reintegrated.
The Chinese proved receptive since they were facing a massive Soviet build-up on their borders, and had also been weakened internally as a result of the Cultural Revolution . They understood that the United States — not the Soviet Union — held the key to China’s reentry into the world community, and that a relaxation of US hostilities would allow rapid normalization of relations with Japan and various other Western nations, culminating in both economic and political strategic benefits.
Kissinger’s “ping – pong” diplomacy was capped by President Nixon’s official visit to Peking in February 1972.
The United States declared that it acknowledges that all Chinese on either side of the Taiwan Strait maintain there is but one China and that Taiwan is a part of China. The United States government does not challenge that position. It reaffirms its interest in a peaceful settlement of the Taiwan question by the Chinese themselves. With this prospect in mind it affirms the ultimate objective of the withdrawal of all US forces and military installations from Taiwan. In the meanwhile, it will progressively reduce its forces and military installations on Taiwan as the tension in the area diminishes.
The Chinese, for their part, did not insist that Peking be declared the legal government of all China, nor did they insist that the US abrogate their 1954 mutual defense treaty with Taiwan.
In actuality, military assistance to the island nation had already been terminated when Peking was admitted to the United Nations in 1971, and the US had then refused direct sales of advanced military equipment.
Afterwards, American security support had focused on helping Taiwan upgrade its own defense industries. From the mid 1970s on, domestic production of jet fighters, helicopters, guided missiles, artillery, and other weaponry increased, often under contract to US manufacturers.