Last Thursday evening, just after Secretary of State Cyrus R. Vance completed a contentious meeting in New York with Soviet Foreign Minister Andrei A. Gromyko, a message from Moscow began clattering across the Pentagon terminal of the U.S.-Soviet hotline.

That message, from Soviet President Leonid I. Brezhnev to President Carter, forecast the final results of three weeks of negotiations about Soviet troops in Cuba.

It also went a long way toward determining the mixed report that President Carter delivered to the American people via television Monday night.

The exchange started a week ago yesterday with a message from Carter to Brezhnev, via the satellite-and-teleprinter hotline, stressing that U.S. objection to the Soviet "combat brigade" was a very serious matter that could adversely affect overall relations of the nuclear superpowers and would be important in the Senate consideration of the strategic arms limitation treaty (SALT II).

Brezhnev's reply made it clear that the Soviets would be of little help in resolving what they believed to be essentially an American political problem, not of their making.

According to official sources familiar with the exchange, the Brezhnev message contained the essence of the Soviet qualifications and assurances that Carter made public Monday night.These fell far short of what Carter needed to settle the issue by diplomatic means.

In essence, the Soviets agreed not to do in the future what the United States claims they have done -- to create a threatening combat presence in Cuba.

Although this was not spelled out in public, the Soviets are reported to have agreed they will not supply airlift or sealift to Soviet forces in Cuba. They apparently agreed not to contradict a U.S. unilateral "understanding" that the Soviet force will not be enlarged or given additional capabilities.

All this is the language of diplomacy rather than the language of politics. Carter faced a serious political problem, partly of his administration's own making. A solution with mirrors and symbolic gestures -- "a nonsolution to a nonevent" as some have called it -- was made necessary by the Soviet refusal to take real action to defuse the issue.

The alternative to this somewhat messy nonsolution, in the face of Soviet uncooperativeness, would have been redoubled U.S. public and private pressure on the Soviets to change their position. Under the circumstances, this would have courted, and probably ensured, an escalating U.S.-Soviet confrontation, with uncertain prospects and grave danger.

Another president might have chosen this route. Carter, whose dedication to nonintervention and nonconfrontation is fundamental, chose a course that is safer internationally but more perilous in the arena of domestic politics.

At home, the unenthusiastic reaction of the Senate does not bode well for the fate of the embattled SALT II. On the other hand, a growing confrontation with the Soviets would have doomed the treaty unless the United States came up the clear winner in a showdown, and perhaps even then.

Abroad, the mild first reaction from Moscow may be a holding posture while definitive attitude takes form in the Kremlin.

It is too soon to say whether the Soviets take Carter's countermeasures as serious actions that they, in turn, must counter. Their considered reaction may be better read in deeds than in words, and deeds take longer to develop.

Carter has given up hope that in the immediate future there will be observable changes by the Soviets in the activity or equipment of the Soviet unit in Cuba. But he retains the hope that, over time, such changes may take place quietly.

Administration officials went out of their way yesterday to reduce Soviet apprehensions about Defense Secretary Harold Brown's forthcoming trip to the People's Republic of China. The leaked news of the trip on Monday seemed to cast it as one of Carter's answers to the Soviet bridgade in Cuba.