In 1977, while the European strategic scene was essentially in repose, the Soviet Union started aiming new, menacing, mobile, triple-warhead SS20 missiles at Western Europe. Its evident purpose was to test the post-Vietnam possibilities of weakening Europe's Atlantic tie. Through two American administrations, a NATO response was shaped and put into effect: to negotiate and, when that failed, to deploy countering missiles. The response had its costs and flaws but -- the essential point -- the Atlantic tie held. Through it all, for eight years, SS20s were being relentlessly wheeled into place at the rate of one a week. Sooner or later, everyone knew, Moscow would have to stop, there being no valid military reason and no political reason, beyond intimidation, to go on.

Now Moscow says it has stopped. The new Soviet leader, Mikhail Gorbachev, announced it Sunday. As everyone expected, he presented the halt as a good- faith moratorium for which Moscow should be recompensed, by November, with a halt to the American deployments. Otherwise, Mr. Gorbachev said, his government will review the moratorium. In something of a similar tease, he held off from public confirmation of a summit with President Reagan.

Some moratorium. It gives the Soviets an advantage in intermediate-range missile warheads on the order of, at this moment, 8 or 10 to 1. Meanwhile, they are working up a new mobile missile. Their plain strategy is to make political capital, especially in Europe, out of the moratorium and out of the familiar, stale calls for a freeze on strategic weapons and for a ban on space weaponry that Mr. Gorbachev also made in his Sunday statement. By this reach for Western opinion, Moscow evidently hopes to improve its bargaining position at the Soviet-American arms control talks under way in Geneva.

In the earlier period, the Soviets went for broke and tried to block American deployments altogether while proceeding with their own. They ended up creating a disparity in the numbers that was bound to be extremely difficult to narrow by negotiation. And in the earlier talks, there was no narrowing. There was only deadlock.

In the talks going on now in Geneva, the administration apparently means to concentrate on reducing the longer-range offensive strategic arms and, meanwhile, to try to fold in the intermediate- range missiles, which are militarily less significant but still of major political importance. The Kremlin is still trying to make the American deployments a wedge between the United States and Europe.

The requirement for the Western allies is unchanged: to continue negotiating on the whole range of strategic weapons with Moscow and to keep it clearly in mind why they resolved to respond to the SS20s in the first place. Those weapons represented an effort to establish an intimidating nuclear presence. They are, in very large numbers, still there.