THERE WAS A TOUCH of cultural irony to the establishment of full relations by the United States and China on Monday.The Chinese, ostensibly the more history-minded of the two countries, ignored the ups and downs of the past and emphasized the political and strategic benefits expected in the future. It was left ot us Americans to look back and to recall that, in President Carter's understated words, "the estrangement of our peoples has sometimes produced misunderstanding, confrontation and enmity." That reflective note is appropriate, we think, at a moment when elation over ending what Mr. Carter termed "a generation of isolation" may be crowding realism, at least temporarily, aside.

The fact is that Monday's deed will deepen but not transform relations between Peking and Washington. The principal reasons are two. On the political side, the United States and China, both intent chiefly on offsetting Soviet power, have been moving toward normalization in plain view for almost a decade. The resultant geoplitical adjustments are already on the way to being made. Then, on the economic side, Peking's poverty will long restrict its capacity to enter the world economy -- specifically, to trade with the United States. It is easier to profess an ideology of modernization than to earn the foreign exchange to make it real. Meaningful oil and mineral exports lie years down the road. Meanwhile, China seeks tens of billions of dollars in low-cost loans.

We note that he assurances on Taiwan that the United States required were no part of the ceremonies on Monday. Yet in itself this is not cause for dismay. Taiwan greeted the new year with a ritual call to reconquer the mainland. But Peking has expressed its respect for Taiwan's economic achievements and its claim on "political autonomy," ended its bombardment of the offshore islands, and let it be known that it was shifting troops from the coast. In knitting these admittedly reversible gestures into a patternof softening bilateral relations with the mainland lies Taiwan's best hope for a secure future over the long term. Its own armed strength and American concern are worth a good deal but its realism will eventually be worth more.

Those Americans who worry about the way the United States is ending its diplomatic and defense ties with Taiwan will do what they can to ensure the administration makes good on its Taiwan pledges. But those Americans would do Taiwan no less a service by commending to it the advantages of adjusting realistically to a new era.