THE Soviet Union has thousands of continent-crossing missiles armed with nuclear warheads ready to launch at the United States - more strategic firepower than ever.But the Soviet threat that President Bush is discussing in most detail here does not concern war but rather how to prevent accidental or unauthorized missile launchings. The military threat the Soviet Union presents as a competitor to the United States and the West has certainly diminished in the past two years. How much still leaves a lot of room for debate. "I think that for at least the rest of this century, the Soviet threat is zilch," says Jenonne Walker, a Carnegie Endowment for International Peace fellow and former arms-control negotiator with the US State Department. The only capacity the Soviets have left to threaten the US militarily is to push the nuclear button, she says, "but there is no conceivable leader who would have an interest in doing so." But Frank Gaffney, director of Center for Security Policy and a former Defense Department official, says, "We still face a formidable Soviet threat." After six years under Mikhail Gorbachev, the Soviet military-industrial system, especially for the strategic systems that directly threaten the US, are running more efficiently than ever, he says. The key to whether the Soviet is a threat in the coming decade and beyond - which is the time horizon of most weapons-building and arms-control decisions - is the outcome of the current power struggle in the Soviet Union. The safest scenario is one where democratic pluralism and economic ties to the West flourish in the Soviet Union, with the central Soviet government maintaining control of international affairs and defense policy for the whole country. White House officials have spoken in favor of such a scenario, where the central government holds enough power to preserve stability. The most dangerous scenario is a takeover of the central government by hard-line communists and the military, perhaps through a coup dtat. The most unpredictable is the Soviet Union breaking apart Yugoslavia-style or erupting in civil war - the danger there being the risk that strategic nuclear missiles would fall into the hands of factional groups. Many American leaders, including Sen. Sam Nunn (D) of Georgia, chairman of the Senate Armed Services Committee, are especially concerned about protecting against rogue launches or accidental missile strikes. The Soviets have improved their fail-safe systems against unauthorized launches in the past few years, so that a missile cannot be launched locally without special codes sent from Moscow. But American systems have been improved even more, and Bush aides said that he and Mr. Gorbachev would discuss these systems this week. Further, the Soviets have removed some missiles deployed in the most contentious regions of their country to prevent their seizure. The deflating of the Soviet threat derives more from changing Soviet intentions and the probability of attack than from loss of capability. Soviet strategic power has grown a steady 3 percent a year under Gorbachev, according to Tyrus W. Cobb, senior associate for political-military affairs at the Center for Strategic and International Studies in Washington. The US has about 12,000 strategic nuclear weapons in missile silos, submarines, and bombers; the Soviets have 11,000. The Strategic Arms Reduction Treaty (START), to be signed tomorrow, will probably bring the Americans to 9,000, the Soviets to 7,000. START will barely inch the world toward greater nuclear safety. If even five Soviet warheads penetrated American defenses, notes Jenonne Walker, they could destroy the Boston-Washington corridor. But since START does require the dismantling of half a Soviet first-strike weapon, the SS-18 missiles, it "helps a very little bit," she says. On other fronts, Soviet strength is diminishing more noticeably. The naval fleet is aging out of competition with the US as a means of projecting forces to distant regions or as a blue-water fighting force, says Dr. Cobb. The big difference in ground forces is that the Soviets have pulled away from the Western front in Europe to a large extent and no longer have the Warsaw Pact to support them there. Many analysts believe that no matter how aggressive the Soviet leadership could become, the country could no longer finance a major rearmament with a badly deteriorating economy. A hard-line government, too, could be too absorbed in trying to consolidate its own power in the Soviet Union to become aggressive abroad. The US and Soviets still have conflicting interests and aspirations in many parts of the world - from Afghanistan to Cuba to Cambodia. If any of these flash points pit the superpowers against each other, says Peter Huessy, senior defense analyst at the Institute for Foreign Policy Analysis, the balance of strategic weapons once again could be called into play. "The cold war is in a different phase," he says. "But it's still there."