Today's Soviet-American agreement to resume substantive arms control negotiations marks a new start in the relationship between the world's two most powerful nations -- a renewal made possible by Moscow's apparent realization that its hard-line stance over NATO missiles in Western Europe had gained it nothing.

Instead of splitting the United States and its allies, the Kremlin had found itself facing a NATO determined to press ahead not only with its missile program but also with substantially strengthening its ground forces.

In turning away from the belligerence and mutual recriminations that have marked the U.S.-Soviet relationship in recent years, Soviet Foreign Minister Andrei Gromyko's effort at damage control sought to shift attention from the areas where Moscow already had stumbled -- the intermediate and strategic weapons talks -- to the Kremlin's new worry, President Reagan's space-defense program.

Gromyko likely will make much of this in the coming days, and the Reagan administration no doubt will emphasize, as Secretary of State George P. Shultz did tonight, that it made firm its insistence that Star Wars research is to continue even though the issue is now on the bargaining table.

On balance, it is the Soviets who seem to have made more concessions to restart the negotiating process which they broke off more than a year ago.

The question is, why did they do that on terms similar to those that Washington proposed last summer and that Moscow rejected out of hand?

Perhaps the most significant Soviet consideration in returning to the arms talks is related to Reagan's Star Wars program.

The Soviets would like to have Reagan abandon the program altogether because the inevitable competition in this high-technology arena would mean a major drain of Soviet resources. If they could not have the American president give up the program altogether, the Soviets would like at least to limit its scope.

In this context they are carefully following the attitude of Washington's European allies toward the Star Wars program.

There are widespread suspicions in Western Europe about this program and its eventual impact on America's commitment to the defense of Europe. In the eyes of many Western Europeans, implementation of the Star Wars program would have the effect of separating the defense of the United States from that of Europe, something the United States has been trying to forestall by deploying Pershing II and cruise missiles in European countries.

Moscow's traditional goal has been just such a decoupling of Western Europe from the United States resulting in a weakening of the western alliance. At this time, and with little prompting from the Kremlin, significant portions of public opinion in Western Europe have been led to view long-term U.S. intentions with suspicion because of the Star Wars program.

Western European leaders have firmly supported the Reagan administration's program of Star Wars research out of a belief that it was the best way to get the Soviets back to the nuclear bargaining table. But they also have made it clear that they want the program used only as a bargaining chip and not implemented.

The resumption of Soviet-American negotiations was expected to be used by Moscow to fuel European concerns about U.S. intentions and thus generate pressures on Washington to limit the scope of its space-weapons plans.

Political observers say another element in Moscow's strategy is to seek to exert influence on U.S. opinion. The president's program is being questioned in the United States, where Congress ultimately has to allocate funding for it.

To what extent the Soviets will be able to play on weaknesses of the western world remains an open question. But it is clear that the end of arms control negotiations had left the Soviet Union not only without a forum in which to seek accommodation with the United States but also with a diminishing leverage on western and American public opinion.

Both sides stressed that negotiations were likely to be tough and complex. But there was satisfaction on both sides about an agreement that seems to restore a degree of basic confidence between the two superpowers as they embark on a third phase of nuclear-arms negotiations.

American diplomats were clearly pleased by the outcome, although they kept their expressions of satisfaction muted.

The Soviets have abandoned their insistence on the removal of U.S. Pershing II and cruise missiles from Western Europe as a prerequisite to the resumption of talks on intermediate and strategic arms. They also formally accepted Reagan's idea for the need for "radical reduction" of nuclear weapons.

In turn, the American side accepted Moscow's insistence for talks on space weapons.

This was the most difficult issue discussed during the past two days here, and its final resolution is marked by a degree of purposeful diplomatic ambiguity to save face all around.

While Moscow previously insisted on the formula focusing on the prevention of the militarization of outer space, the communique addresses efforts to prevent the arms race in space. This seems to leave some room open for Reagan's planned research in antiballistic defense systems.

In turn, the United States recognized an interrelationship between progress in space weapons negotiations and those dealing with strategic- and medium-range nuclear arms. The nature of this relationship was less vague and presumably will be defined during the course of the talks.

In doing so, however, the United States has broadened the area of space weapons to include land-based systems with a space potential. This indicates that Soviet antiballistic and antiaircraft facilities would fall into this category.

There is little doubt that the agreement today was a personal success for Shultz and his negotiating skills.

A fresh start in the dialogue on arms control was based on the secretary's idea to offer Gromyko an entirely new approach, which was dubbed "umbrella talks," to include strategic, medium-range, and space weapons.