PRESIDENT REAGAN handled the Taiwan FX question well. On the basis of his long admiration for the Republic of China, a country which has become a cause to many on his party's right, the president could have been expected to provide an advanced FX warplane forthwith. Taiwan's FX request was being presented as a test of both his constancy and his ideological mettle. But wisely, Mr. Reagan finessed the question. He accepted this country's obligation, reinforced by legislation after relations with the People's Republic of China were "normalized" in 1978, to continue providing means of defense to Taiwan. He found, however, that Taiwan's defensive requirements could be met by equipment without the militarily and politically provocative quality of an FX.

Why would the FX in little Taiwan's hands be provocative? For reasons presumably arising from still-dim considerations of Politburo politics, the Chinese last year abandoned their agreement-to- disagree on American arms sales to Taiwan and opened a major campaign to induce President Reagan to stop selling arms altogether--and certainly not to provide an FX. In the case of Taiwan, the stakes were largely ideological and political. In the case of Peking, they were largely geopolitical. China threatened, even at substantial cost to itself, to pull back from Washington and to loosen the ring of containment Mr. Reagan has been trying to tighten around Moscow.

As foolish as it would have been to make light of these threats, it would have been unthinkable to abandon an old friend. Mr. Reagan did neither, though some of his ideological constituents are not so sure of this. His military advisers found, as experts outside the administration already had, that Taiwan's hardware needs are relatively modest. The president decided to continue fulfilling them and so informed Peking.

It surely will not help Taiwan to advertise whatever disappointment it may feel at not getting the FX. Peking has chosen to advertise its disappointment that any sale at all was made, but it seems to be doing so with restraint. It calls the United States, on this issue, "obstinate." One would hope so.