What have the United States and China gained from the flowering of their relations since President Nixon's visit to Peking ten years ago?
What does each stand to lose should relations deteriorate because of the dispute over American arms sales to Taiwan?
For both countries, the greatest gain has been the change in the global strategic balance and the added security each enjoys because of this change.
Soviet expansionism has not been checked. But from the American or West European viewpoint, the alarming Soviet military buildup in Europe would be far more threatening were Moscow not forced to divert perhaps a quarter of its military resources to its long border with China. If China had improved relations with the Soviet Union instead of with the US, the Soviet Union would have been freer to reinforce its NATO front by shuttling military forces from the China front.
From China's viewpoint, Moscow's constant fear of a two-front war, including Europe as well as China, keeps it from ever being able to mass its forces for a strike against, say, northeast China. Moscow might have attacked China during Peking's February-March 1979 ''defensive offensive'' against Vietnam, had China been as isolated as during the Sino-Soviet clashes of the 1960's.
For Washington, one important bonus of the Sino-American relationship has been the calming, stabilizing effect it has had in the Far East. Japan is far happier to have China and the United States friends than enemies. Sino-American friendship means that Japan's own security treaty with the United States is directed almost exclusively against the Soviet Union.
On the Korean peninsula, China's non-confrontational relationship with the US makes it more likely that Peking would want to restrain North Korea from military action against US-backed South Korea.
Both China and the United States have benefited from increased trade. China has become one of the largest customers for American agricultural products, while the United States is an increasingly valuable market for Chinese textiles and other manufactured goods. Last year total two-way trade reached $5.5 billion.
Washington and Peking have signed over 30 agreements during the past three years, from a consular treaty to scientific and technical exchanges. Eight thousand Chinese students, 3,000 of them sponsored by the Chinese government, are studying in the United States.
All cooperation would not necessarily come to an immediate grinding halt if current delicate negotiations over United States arms sales to Taiwan fail and Sino-American diplomatic relations are downgraded.
But such an event would mean that for the first time in 10 years the positive direction of the relationship would be reversed. The two countries are not yet so solidly linked or free of past suspicions that ''dark demons'' might not be unleashed, in the words of one diplomatic source.
In Southeast Asia, where Moscow has been actively supporting Vietnam against China, cooler Sino-American relations might reduce Vietnam's fear of China. This could make it easier for Vietnam to attack Thailand in order to end the Chinese-supported Khmer Rouge guerrilla resistance to Vietnam's military occupation of Kampuchea (Cambodia). Vietnam so far has been deterred from major attacks into Thailand out of fear that China might again attack across Vietnam's northern border.
But a China again estranged from the United States might feel so vulnerable to Soviet invasion that it would not dare to again attack Vietnam.
For the United States, new strains in the security relationship with Japan might show up. The Korean peninsula could destabilize.
The greatest loser, ironically, might be Taiwan. For if China and the United States come to a parting of the ways over Taiwan, tension in the Taiwan Strait, now almost non-existent, could quickly revive. Confidence in Taiwan as a place in which to invest and with which to do business would inevitably be affected, in the United States, Western Europe, and Japan.
Taiwan's economic expansion upon which its present prosperity is based would come to an end. And the United States, as Taiwan's only source of military supplies, would have to shoulder much of the costs of Taiwan's added defense needs.
Western diplomats here sometimes differ on the likely consequences of a possible deterioration of Sino-American relations. But they generally concur that the negative consequences would probably far outweigh the advantages for either Peking or Washington from standing firm on what each side considers to be a matter of principle.