To reduce the danger, Part 3

Successful bridging of the chasm of fear and hostility between Moscow and Washington has occurred twice under special circumstances. The circumstances should be noticed and identified because they provide guidance to those now seeking to build new bridges in the hope of reducing the danger of nuclear war.

The first was during World War II when a common danger generated a willingness on the part of both to cooperate with each other. That cooperation was never easy. Mutual suspicion plagued the course. A highlight was the suspicion, on the part of both, that the other might conclude a separate peace with Hitler. But both stayed the course and continued the collaboration until final military victory.

The second was during the Nixon-Kissinger era when China broke away from its alliance with Moscow and entered into new and cooperative relations with the United States. This adjusted the power balance in the world away from Moscow and toward Washington. The result was a short period of so-called detente. There is one survival, the SALT 1 agreement, which still limits the total number of intercontinental ballistic missiles in the US and Soviet arsenals.

It is thanks to SALT 1 that the roughly 8,000 nuclear warheads, which Moscow and Washington each have and could hurl at each other, stay at roughly that figure and are not higher. Dangerous as is the present balance of terror, it would presumably be even more dangerous and more expensive without SALT 1 - and SALT 2, which remains in force even if not ratified by the US.

In each case Moscow entered into a limited and uneasy cooperation with the US when events were less favorable to the Soviet Union.

The Soviet Union came very near to being overrun by Hitler's armies in the German invasion of 1941. Stalin himself left Moscow. Moscow would probably have fallen to the Germans had not winter set in suddenly and fiercely on a German Army unprepared for that kind of cold. It was the moment of greatest danger in the story of the Soviet Union. Moscow was willing in that extremity to accept help from the ''capitalist'' outside world.

In 1972 Richard Nixon went to Moscow and received a relatively friendly welcome. The Soviet condition was less parlous than it had been in 1941, but Moscow was having troubles and knew that it could improve its condition if it could obtain access to the granaries and the higher technologies of the West. Its diplomacy had failed with China and was in trouble with Egypt. Its agriculture was lagging. Its industries were falling behind the West.

President Nixon had first gone to Peking in February, then to Moscow in May. By July he had approved a grain sale to the Soviets worth over $750 million for a three-year period. But the Soviets took almost all of the three-year quota during the first year. They were in serious need of that grain.

Moscow's inclinations are to act alone and trust no other country. During the Brezhnev years it experimented with the possibility of an easier and more cooperative arrangement with the US. But it did not last through the Carter administration.

In other words, bridge building with the Soviets is easiest when they are in trouble and in need of help. They were in desperate military need in 1941. They were in diplomatic and economic trouble in 1972. Right now there is no comparable need to stimulate them toward new cooperation with Washington. The only danger they share in common with Washington today is the danger from the existence of those two massive arsenals of nuclear weapons.

Instead of needing help from Washington, through improved relations, they seem at the moment to find an advantage to themselves in strained relations.

The essential world power situation today is the alliance between the US and Western Europe on one side of the Soviet Union and an association between the US and China and Japan on the other side. Moscow is unlikely to feel secure so long as that condition prevails. The main object of its diplomacy is to break up the NATO alliance and the US association with Japan and China.

Would a revival of detente promote Moscow's purpose, or work against it? The Kremlin leaders probably think that a tense US-USSR relationship helps their purpose by frightening America's allies and friends. Also, they seem to have some reluctance to try doing business with a person who calls them the ''focus of evil'' in the world.

Still, a new arms limitation agreement remains possible.

To be concluded on Thursday