SIXTEEN YEARS AGO this week, on Nov. 19, 1969, U.S. and Soviet diplomats sat down in Helsinki to begin negotiating a limit on the most powerful arsenals in history: strategic nuclear weapons capable of swiftly and utterly destroying most of mankind. This week in Geneva, the summit meeting of President Reagan and Soviet leader Mikhail Gorbachev will go a long way toward determining how -- and perhaps whether -- this extraordinary effort to curb one anothers' weapons through diplomacy will continue.

The enterprise known as "arms control" is at a crucial hour.

If Reagan and Gorbachev can make even a start on new understandings, their Geneva summit could endorse and possibly advance the drive for significant arms cuts. Failure to find common ground, however, could deal the final blow to several existing arms accords, which cling to life today by only a slender thread.

Unlike the 1972 Moscow summit when President Richard M. Nixon signed the SALT I Treaty and the Antiballistic Missile Treaty, the 1974 Vladivostok summit when President Gerald R. Ford approved an interim accord on strategic arms and the 1979 Vienna summit when President Jimmy Carter signed the SALT II Treaty, the stage has not been set this week in Geneva for Reagan to ratify or negotiate a new U.S.-Soviet arms agreement.

Nevertheless, underlying conditions seem favorable for forward motion. Arms negotiations resumed last March after a year-long Soviet walkout. Within the last two months, both sides have proposed unprecedented cuts of 50 percent in strategic nuclear weapons -- though on very different bases -- and major cuts in intermediate-range nuclear forces in Europe. The discussions, at least, advance previous accords, which sought to cap future growth in military programs rather than deeply cut existing arsenals.

Economically, the staggering cost of the arms race, which has continued at an alarming clip despite past arms-control achievements, has become a heavy burden for both superpowers.

Any relief from these soaring costs would be politically welcome in Washington and Moscow, where for the first time in years leaders of both nations simultaneously appear secure in their jobs and physically fit, with sufficient authority and tenure to conclude arms agreements and make them stick.

Those positive signs notwithstanding, relations between the two nations since the 1979 Vienna summit have sunk to the lowest point since the heyday of the Cold War in the 1950s. This underlying hostility has deeply affected arms control.

Today, good faith and understanding have been virtually abandoned in negotiating and policing of arms-limitation agreements. Instead, ironclad monitoring arrangements for compliance are deemed essential, particularly because of U.S. charges that the Soviets have cheated on earlier treaties. But verification through satellites and other technical measures, which made previous agreements politically acceptable in the United States, is becoming increasingly difficult as the two nations move toward smaller, more mobile, easily concealed nuclear weapons.

The most imposing hurdle at the moment, however, is Reagan's anti-missile defense plan, the Strategic Defense Initiative. Soviet officials and many American critics have said Reagan can have an unrestrained "Star Wars" program or arms agreements to reduce offensive nuclear missiles but not both.

Among those holding this view is former defense secretary Harold Brown, a nuclear physicist with technical and political experience in strategic arms. "In my judgment, it is not compatible with offensive limits to have the expectation of a defensive deployment" such as the SDI to stop offensive weapons, Brown said in an interview.

Introduction of even "the potential" for anti-missile defensive systems, sharply limited in quantity and kind under the 1972 ABM Treaty, "complicates matters and makes it very hard to reduce offensive forces. In my judgment, neither side is going to say, 'The other side may put in a big defense. Let's reduce our offense and see whether maybe we won't be able to get through,' " Brown said.

"To the extent that the United States is or appears to be committed to an SDI program that implies a significant likelihood of deployment of an urban-industrial defense, I think arms control is not going to be feasible," he added. "Indeed, I think existing agreements will then either erode or be abandoned."

Gorbachev, in an interview early this fall with Time magazine, said a U.S.-Soviet agreement to "prevent an arms race in space" by curbing the SDI is a necessary condition for reductions of offensive nuclear arms. "If the present U.S. position on space weapons is its last word, the Geneva negotiations will lose all sense," the Soviet leader said in a formulation that seemed to suggest, against the expectation of most western observers, that the Soviet Union might break off the talks.

The official U.S. position that offensive-arms cuts can be negotiated even as the SDI proceeds unrestrained is a minority view among arms experts. A proponent of the position, Ambassador Edward L. Rowny, a senior U.S. arms negotiations adviser, predicted that "when the Soviets realize that we are not going to use SDI as a bargaining chip -- and I don't think we will do this -- the Soviets may move somewhat toward offensive cuts." He added that "the time is not yet" for this Soviet turnabout.

Ambassador Paul H. Nitze, the other senior U.S. arms-negotiations adviser, drafted a "strategic concept" for the administration earlier this year calling for "radical reduction" in offensive nuclear arms and "stabilization of the relationship between offensive and defensive nuclear arms, whether on Earth or in space" during a 10-year transition period before it becomes known whether the SDI can succeed.

Nitze said "it's an oversimplified logic" to argue that there is little chance for strategic cuts without early agreement on the SDI.

"I've seen many unexpected things happen. I don't think it is so easily predictable," said Nitze, who drafted the first U.S. strategic plan for dealing with the Soviets in 1950 and has been involved in strategic thinking ever since, either as policy-maker or critic.

As Reagan and Gorbachev sit down in Geneva, the clock will be ticking on the fate of the 1979 SALT II Treaty, the most important treaty restraint on offensive nuclear programs. As a private citizen, Reagan opposed the treaty, but as president he, like the Soviet leadership, pledged not to undercut it although the pact was never ratified by the Senate. However, SALT II expires Dec. 31, and what happens then is still uncertain.

The 1972 Anti-Ballistic Missile Treaty, another crucial arms-control agreement, was recently "reinterpreted" by the Reagan administration and is jeopardized by the race for a space-based ABM.

The Soviets in recent months have proposed a series of temporary unilateral bans on various weapons activities. Moscow's moratorium on deployment of additional intermediate-range nuclear missiles in Europe expires this month, and a moratorium on underground nuclear weapons testing expires Dec. 31.

The Soviets have indicated that they no longer feel bound by a moratorium on antisatellite weapons testing after a U.S. test in September, but no new Soviet ASAT test has been conducted.

Given the inability through past agreements to curb increasingly powerful and accurate nuclear weapons, a stalemate in Geneva leading to further erosion of existing agreements might have only limited military effect. Of equal or greater significance, however, is the potential political impact.

Arms-control negotiations have been a forum for U.S. and Soviet discussions even in the worst of times. They symbolize a common interest of the two leaderships in avoiding a war that could end all human life. For arms control to lose ground or collapse, after 16 years of effort, would send a powerful message that mankind, ever hopeful about the future, is loathe to hear.