FOR YEARS the People's Republic of China complained about the United States' insistence on providing for the defense of Taiwan. Now it has gone beyond complaining. It threatens, if the administration sells any kind of new aircraft at all to Taiwan, to "pull back" (in unspecified ways) from its developing relationship with Washington. Whatever the reason for this escalation--it is, as is usual in Chinese affairs, a matter of much conjecture--it leaves Mr. Reagan with a real problem. We think there are two considerations that must guide him through it.

First, he has to do what is necessary for the defense of Taiwan. The long American association with Taiwan demands it. A due respect for the law demands it--the Taiwan Relations Act of 1978 mandates an American defense relationship. The requirement to demonstrate that American commitments are meaningful demands it. Peking has chosen, for its own reasons, to alter the pattern set in the nine years following the Shanghai Communiqu,e, which established the basic terms of Sino-American relations, and to add a new layer of threat. But that cannot be the determining factor for the United States. The commitment to the well- being of Taiwan is unchanged.

If Mr. Reagan must do what is necessary for the defense of Taiwan, however, he must do only what is necessary. That obliges him to make the most careful judgment of its actual security requirements. It is, as far as we know, the uncontested judgment of professionals that Taiwan has never been less threatened in a military sense by the People's Republic. The Nationalists, having been routed on the mainland in the Chinese civil war, could take over the island where they consolidated their residual power, but the Communists still utterly lack that capability. Peking is embarked on a long-term diplomatic strategy of reunification to replace the "liberation" strategy that, fortunately, failed. In these circumstances, whether Taiwan needs a new aircraft apart from the F5E, which it has co-produced for years, becomes a narrow technical question.

If a good military case can be made for selling a new airplane, then it must be sold, no matter that it leaves the administration under a difficult burden to explain to Peking the normal and unprovocative quality of the transaction. If it is merely a situation in which Taiwan wants a new plane for which there is a political rationale but not a strong security rationale, then the old plane will do. In either case, neither Peking nor Taiwan should be left in any doubt as to where American foreign policy is made--in Washington.