WE DON'T PRETEND to comprehend the full implications of the agreement that has now been reached to establish full diplomatic relations between the United States and the People's Republic of China. Surely the test of it will lie not just in some larger geopolitical striking of balances between the world's great powers but also in the very specific matter of the fate of 18 million people on Taiwan. The critics have already zerod in on the Taiwan question, and with good reason: The American commitment to the Taiwanese is of long standing. It is moral, not just legal. And other countries heavily dependent upon America's good will and good word have reason to concern themselves with the manner in which this country, in its future and expanded relations with Peking, holds fast to its obligations to Taiwan.

That said, it needs to be quickly added that there is a compelling logic to what President Carter has done to establish full diplomatic relations between Peking and Washington. This is basically what was contemplated when President Nixon made the first big breakthrough in 1972 and President Ford followed it up with his own visit to Peking. The only questions were when, on what terms, and to waht purpose. So we begin with: Why now? And the answer seems to be that, from Jimmy Carter's point of view, an improved opportunity was there. He had let it be known last year that he was ready to proceed. But only in the past few months did it become apparent that authority in China seemed to be passing into the hands of a more outward-looking leadership, willing to modify its harsh terms vis-a-vis Taiwan in return for a rich variety of relationship with the West.

And so, in hard bargaining over the past few weeks, a deal was struck. On the issue of Taiwan, Peking's concessions may not look impressive. The United States will "terminate" relations at the end of a year in accordance with a treaty provision, rather than abrogate the treaty. There is provision for American arms transfers, even after the termination of official relations. Cultural, commercial and even consular relations would be continued under legislation that the administration will send to Congress next year. And there is at least some sort of implicit acknowledgment on the part of Peking that the United States, while accepting that the resolution of the Taiwan issue is to be an internal concern of the People's Republic, does not expect that the jurisdictional question will be resolved by force.

The value of vitality of any of these conditions and terms depends, of course, on what it is the Chinese really want-and how badly they want it. The calculation the president clearly made was that the People's Republic is serious about its new desire for strengthened economic and political relations with the West in general and the United States in particular. And if this is so, it would seem to follow that Peking will not deal with Taiwan in a way that it must know would jeopardize these expanded relations. Mr. Carter must also have obviously calculated that a strengthened bilateral relationship with the Chinese would be an extremely useful element in his dealings with the Soviet Union.

It's a gamble, and not just in highly complex geopolitical terms. It is also a very big gamble in domestic political terms, at a time when there is considerable anxiety, and not just on the far right, about Jimmy Carter's handling of SALT negotiations and his capacity to manage a SALT II agreement. So this took some political courage as well as adroit diplomacy on Mr. Carter's part. Time will tell whether it pays off.