SUDDENLY, the showdown is over. The United States and the Soviet Union are carefully, gingerly pulling the knives away from each other's throats and beginning to be civil. They've announced talks between Secretary of State George P. Schultz and Foreign Minister Andrei Gromyko to explore "the entire complex of questions concerning nuclear and space weapons." President Reagan's national security adviser, Robert C. McFarlane, says the United States is prepared to be "flexible and constructive" in these talks. The head of the Soviets' American Department is caught singing. "We're all in this together" at the ambassador's house on Thanksgiving.

Vieturn to detente? A nuclear freeze? Some kind of brake on the arms race? Hardly. The latest love in between? Washington and Moscow has about the same chance of breaking into open arms control as did the four years of deep freeze that characterized relations during the first term of the Reagan Administration: nil. All the soothing words enunciated in last week's rush to rapprochement cannot obfuscate the immense underlying obstacles to any meaningful agreement given the current cast of characters in the White House.

*The Fine Print. The task of forging arms control agreements today may well be beyond the ken of any administration, let alone one ill-disposed to the things. The arms race, having spiralled nonstop since World War II, is now prepped to launch into an entirely new level of technology, which is destined to leave the current arsenals--and hence arsenal-limiting ideas--obsolete. Right now, nobody's even sure of a framework to discuss the plethora of land, sea, and air nukes. The likelihood that even dedicated arms controllers could limit this plethora in the next four years is small indeed.

*The Smarts. The trouble in this administration is no one's got 'em. The lack of expertise on arms matters in Washington is approaching the nadir of the post-war world, a depressing trend made seary by the detachment of the President on the issue. The President's three top foreign policy advisers--the secretaries of State and Defense and his national security adviser--are all neophytes in the arcane business of nuclear weapons, leaving the nuts and bolts of the subject to second-level bureaucrats. Historically, every major Soviet American agreement has been carried forward through the initiative of the highest officials in the government; without knowledge and leadership at the top, there will be no arms control.

*The Pentagon Bunker. The lack of expertise in the government is complicated by the fact that the official with the most knowledge about nukes is dead set against limiting them. Richard Perle, the assistant secretary of defense for international security policy, is more than capable of waging four more years of guerilla warfare against any plan for accomodation with the Soviets. Unless he and his patron, the Defense Secretary Caspar W. Weinberger '38, can be forced from office, moderates in the Administration will scarcely have time to think of possible agreements, because they will be too busy lobbing mortar over the Potomac.

*Checking on Cheating. The Administration has been making lots of noise recently about Soviet violations of existing arms treaties, but you can be sure that if these charges were really serious, there's no way these confirmed anti-commies would even be approaching the negotiating table. Nonetheless, one has to wonder how the White House can escape their past rhetoric on the issue. Unless the Soviets agree to on-site inspection--unlikely in the extreme--the Reaganites will have to accept that there is value to an arms control agreement with less rigid guidelines on this issue. They show no signs of doing this.

*Where's the Plan? As Robert Kaiser let out of the bag in Sunday's Washington Post, the biggest open secret on Capitol Hill is that the Reagan Administration has no national security policy, save maybe to spend more money on weapons. Reagan and his cronies, for all their hot air and good intentions--and finally there may well be good intentions--haven't the foggiest notion of what they are going to say once they reach the negotiating table in January. There are no indications that the Administration has rethought any of its basic negotiating positions, especially its refusal to countenance any cuts in its air or sea-based arsenals in return for Soviet cuts in land missiles.

THE ADMINISTRATION'S FAILURE to address these fundamental stumbling blocks is all the more depressing given what should be a fairly propitious atmosphere for an arms control agreement. All signs indicate that the Soviets have not taken lightly the unremitting Reagan arms build-up, and are now especially scared of his proposals to extend the arms race to outer space, where American technological capabilities far outstrip the Soviets'. They now know they have four more years of Reagan and seem prepared to deal.

Which is not to absolve the Reagan Administration of blame for its escalation of the military build-up, which has distorted the economy and brought no appreciable increase in security in today's nuke-dominated world. The words unclear superiority simply have no meaning in international affairs any more, and the failure to yet understand this is perhaps the most disturbing feature of Reagan's bankrupt security policy. There is no clear way to arms control, especially if there is no clear will.