China has managed to win through 10 months of hard bargaining with the United States something that has eluded it in 33 years of civil war with the Nationalist government on Taiwan.

Through the Sino-American communique issued yesterday, China's Communist leaders landed the prize of inevitability in their long struggle to reincorporate the tiny, capitalist island of 18 million people.

Taiwan's Nationalists have been able to postpone indefinitely the painful reunification issue for more than three decades by relying on an unlimited U.S. military commitment as a guarantee of their right to choose. Taipei's battle cry -- "no negotiations, no compromise, no contacts" -- has seemed to predestine a permanent separation of the two Chinese lands.

Now that Washington has pledged to reduce gradually its arms shipments and effectively freeze their quality at current levels, Nationalist officials are certain to face the issue with a greater sense of urgency.

"Peking can just let time run its course," observed a European diplomat. "The pressure will be on Taiwan from now on. This agreement is like one with a time bomb underneath it."

Militarily, the communique seems to all but ensure Peking of eventual air superiority over the Taiwan Strait, which is considered vital for a successful amphibious landing on the island. Washington now is bound to restrict future aircraft sales to the F5E jet fighter that has been supplying Taiwan for years.

Although the F5E can now outfly the most advanced Chinese aircraft, according to Western military sources, mainland technicians are developing a new jet fighter with a Rolls Royce Spey engine that would make it more of a match for the American-built plane.

China is believed to be having difficulty building a suitable airframe for the new aircraft and is several years away from production, military analysts said.

"It's inevitable in time that the Chinese will advance to a position superior to Taiwan if Taiwan has no outside help," a Western analyst predicted. "Taiwan will really start to worry when it sees the next generation of Chinese aircraft."

According to U.S. Embassy sources in Peking who helped to draft the communique, the agreement was designed to reduce Taiwan's security worries.

The reason is Peking's declaration that its efforts to settle the reunification question peacefully represent a "fundamental policy" of the Communist government. Although this codifies a policy China has pursued for three years, U.S. officials said, the statement has a more binding effect as a part of the communique.

Despite Peking's refusal to link progress toward reunification with future levels of U.S. arms sales to Taiwan, American officials said the agreement at least tacitly obligates China to avoid force if Washington is to live up to its promises.

"Gradual reduction of arms sales . . . reflects our assessment of Taiwan's future defense needs," a U.S. Embassy source explained. "That assessment is directly influenced by our expectation that Chi/na's approach to Taiwan will continue to be peaceful.

"Should China depart from that approach, our assessment of Taiwan's defense needs naturally would have to be reviewed."

Taipei now appears to have air superiority, but it is difficult to say for how long, the U.S. diplomat said. Washington is confident that it can provide Taiwan with "sufficient defense capability" within the limits in the communique, he said.

He said the agreement is aimed at enhancing Taiwan's well-being, not compromising its security or compelling Nationalist leaders to bargain for their future with their old Communist foes.

If Taipei still feels threatened, he said, it is free to look for weapons outside the United States.

That suggestion, however, seems to ignore the political impact of the communique, which is to isolate Taiwan further.

Despite losing political recognition by most world capitals, Taipei has managed to maintain commercial ties to many nations. But few foreign governments would be willing to risk Peking's scorn to sell arms to an island it considers part of its territory.

When Washington shifted its recognition from Taipei to Peking in 1979, it reserved the right to sell Taiwan weapons despite China's protests. Then, Congress passed a law committing the United States to provide armaments necessary for the island's defense.

But the United States agreed in the communique to place limits on its commitment in the face of Chinese threats to downgrade diplomatic relations.

"If the Americans with all their muscle give in to Peking, who else would dare" sell arms to Taiwan? a West European envoy asked.

Diplomatically, isolating Taiwan and depriving it of weapons has been a strategy of Peking, which believes such tactics will convince the Nationalists to give up their claim to be the legitimate government of China and to consider reunification.

China has offered several inducements, including a promise to place Nationalist officials in leadership posts of a reunified nation while allowing Taiwan to retain its life style and capitalist economy.

Taiwanese leaders have called the offers "sugar-coated poison" and demanded that Peking renounce communism as a condition for talks.