JUST this summer it seemed big arms control negotiations were a thing of the past. The START treaty on long-range weaponry had taken nine years and hundreds of pages to complete.The conventional wisdom was that both superpowers were too tired of haggling to bring up major new arms deals, at least for a while. What a difference 12 days make. In barely more time than it takes to turn on TV lights, Presidents Bush and Gorbachev have between them decreed the virtual extinction of short-range tactical nuclear weaponry. What's more, they've opened wide the door to talks on some of their most intractable nuclear differences, including the question of "star wars" strategic defenses. Arms negotiators are not yet brushing off their pinstripes and making plane reservations for Geneva. But it's clear that for the near future arms control is once again playing a central role in the relationship between the United States and Soviet Union. After a US delegation headed by Undersecretary of State Reginald Bartholomew returns Oct. 9 from its trip to Moscow, d expect a flurry of activity" in the halls of the Pentagon and State Department in preparation for new kinds of talks, says a knowledgeable US official. Whether President Bush intended to set off such a chain of events when he announced his unilateral arms proposals on Sept. 27 isn't clear. Gorbachev has embraced Bush's approach with enthusiasm, however. Undoubtedly one reason for the response is that Gorbachev knows that, like Bush, foreign affairs is an area in which he does best. And considering the political situation in Moscow, anything he can do to look forceful will be of immense help. "One of the consequences of the Bush initiative is that it tends to elevate the status of Mr. Gorbachev and tends to elevate the status of the central government," says Max Kampleman, a chief US arms negotiator from 1985 to 1989. Gorbachev's detailed response to Bush's proposals contains much agreement on the central question of tactical nuclear weapons. The Soviet leader pledged to unilaterally eliminate nuclear artillery and short-range rockets, as Bush had. This is no surprise, as the Soviets have long called for pulling tactical nuclear arms out of Europe. But in the first of what might be called the "see you, and raise your bid" aspects of Gorbachev's outline, the Soviet leader also called on the US to negotiate a withdrawal of the last remaining kinds of tactical nukes: atom bombs for tactical fighter-bombers. In his own speech Bush had made a point of emphasizing that NATO wanted to retain these weapons. Until now tactical nuclear bombs have been a part of the NATO deterrent that was relatively uncontroversial with European publics. "Now they're certain to take a lot of heat," says David Shorr, associate director of the British American Security Information Council, an arms control group. A second part of Gorbachev's proposal that raises the arms control ante deals with the centerpiece of nuclear arsenals: long-range, or strategic, weapons. In his speech, President Bush had called for a ban on long-range multiwarhead land-based missiles. Since this is a type of weapon in which the Soviets are superior to the US, the Bush proposal was widely seen as a negotiating gambit. Gorbachev simply ignored it. Instead, he called for "intensive negotiations" on all strategic weapons with a goal "approximately to halve them" from the levels called for in the just-signed START agreement. Gorbachev coupled this with, among other things, the news that the Soviet Union would give up the longstanding superpower need to match each other warhead for warhead. The Soviet military instead will unilaterally keep its strategic arsenal at 5,000 warheads, rather than the approximate 6,000 warheads allowed by the START pact. It remains to be seen whether Bush will agree to what would in essence be fast-track START 2 negotiations. But it seems likely that serious follow-on talks will occur in a third major category: defense in space. The Bush administration has long called for some kind of mutual US-Soviet deployment of strategic defenses. The president repeated that call in his latest proposals, saying the defenses should focus on guarding against rogue launch or third-world threats. Somewhat surprisingly, Gorbachev agreed to consider nonnuclear defense-system proposals. That means the moribund Defense and space talks in Geneva will gather new energy. Gorbachev "essentially said he is willing to listen and that is a change," says a US official. One area the US is still studying its reply is nuclear testing. A test ban has been one of Gorbachev's pet projects almost since he assumed power, and in his arms speech he announced a one-year testing moratorium. The US has not responded in kind to past Soviet testing pauses and has given little hint what it will do this time. Mr. Kampleman, for one, believes that some kind of testing limits are "much more feasible" today because of the improved political climate. All in all, two weeks of unilateral arms proposals have made striking progress in a number of areas. But it seems in many important areas the work may just be beginning. "It does look like they will need traditional arms control negotiations," says Mr. Shorr.