THE Soviet Union and the United States are on increasingly amicable terms, and the Kremlin is preoccupied with economic and political reform. So why is a treaty reducing the numbers of nuclear warheads aimed at each other so important?That question could work against quick ratification of the Strategic Arms Limitation Treaty awaiting the signatures of Presidents Bush and Gorbachev at the Moscow summit, July 30 and 31. It could also hurt chances for future arms control building on START. One answer lies in the treaty's obvious strengths, which include: (1) the first significant cutback in long-range nuclear forces (by roughly 30 percent, slightly more on the Soviet side); (2) a reduction by almost 50 percent in the Soviets' most powerful weapon, the SS-18 missile; (3) establishment of what the experts call a "nuclear management regime," embracing methods of inspection and notification designed to ensure that the pact is honored. Each side will acquire an enhanced understanding of the other's military operations. This is particularly useful in light of the Soviets' current instability. Beyond technical gains, START demonstrates that the world's nuclear superpowers are serious about arms control. Apprehension is growing over the proliferation of long-range missiles and weapons of mass destruction. In 1995 the nonproliferation treaty will come up for review. Regional powers eyeing membership in the nuclear club need a good example. Critics of the treaty will speak up both in Washington and Moscow. Some military thinkers are dead-set against sheathing the nuclear sword - even part-way. Distrust runs deep. Others will point out that START really doesn't get rid of any warheads or missiles - that only the silos and other launchers will actually be destroyed under the treaty. Such arguments shouldn't blur the central fact: that after a decade of trying, nuclear warmaking ability will for the first time be reduced. START should provide a basis for deeper cuts in the future. Complicating factors, like the push toward strategic defense systems, will have to be sorted out. But START is a solid start.