What is the proper response to the dramatic new peace offensive that Soviet President Mikhail Gorbachev launched Wednesday at the United Nations?

Keep in mind that the distrust developed through decades of discord cannot be quickly dismantled, that dramatic changes effected by the new Soviet chief could be reversed by a shift in the Kremlin's leadership, and that what counts is not what Gorbachev promises but what he actually delivers.But after such reservations have been raised, the fact remains that the Gorbachev speech represents such a potentially promising improvement in relations between the super powers that the Soviet shift is bound to create excitement and raise expectations around the world.

In essence, Gorbachev backed up a stirring appeal for greater world unity by announcing the Soviet troop strength would be reduced by half a million over the next two years. The troop cuts would be accompanied by reductions in Soviet tank divisions and other conventional weapons in Eastern Europe and along Russia's border with China.

Up to a point, it's easy to raise reservations about these moves.

Though the military cutbacks are being made unilaterally, the Soviets aren't acting out of unalloyed idealism but are serving their own interests. The announcement generates plenty of favorable propaganda for Moscow, improves the atmosphere for next year's Sino-Soviet summit meeting, and enables Russia to divert money from arms to domestic programs and maybe even balance its budget. The Kremlin recently admitted that for the first time it is facing a deficit.

Even with the troop reductions, Russia still will have an estimated five-million men and women in uniform - and a clear advantage over allied forces in Western Europe. That means NATO still can't afford to lower its guard.

Moreover, despite Gorbachev's promise Wednesday of more humane treatment of Jews seeking to leave Russia, the fact remains that free emigration, free association, free worship, and free speech are still a long way from being won in the Soviet Union.

When all such reservations have been recited, however, the fact remains that changes as dramatic as the ones outlined by Gorbachev this week would have been unthinkable only a couple of years ago. Consequently, the Gorbachev announcement should give new impetus to the long-stalled talks on scaling down the Soviet and U.S. forces facing each other in Europe.

In short, Gorbachev has made an important gesture of possibly historic proportions. The challenge now is for the allies to press the Kremlin to maintain the momentum toward improving East-West relations. But those relations never will be all that they can and should be until Russia makes big breakthroughs toward improving human rights, too.