LIKE CLOCKWORK, the Soviet public relations apparat is quietly pushing the approved explanation of why the Kremlin invaded Afghanistan. It happens after every Kremlin-created crisis just as the rest of the world, especially the part of it open to political debate, is recovering from the initial shock and trying to figure out where to go next. Needless to say, Moscow does not speculate, as Americans do, whether the invasion was meant to add a province to the empire, or to redeem a failing political investment, or to muffle Moslem restiveness that might seep back through the border, or to move toward a warm-water port or Persian Gulf oil, or to flex Soviet muscle and test the West, or what have you. Most Westerners are still trying a little this and that at this familiar smorgasbord of possible Soviet motivations. Moscow offers an explanation of its own.

The first and simple part of it is, of course, that whatever happened is the United States' fault. It this instance, Jimmy Carter's zigzagging is cited, the delay on SALT, the non-delivery on trade, the defense increases, the missile program in Europe and the military programs in the Middle East, the joining with China to "encircle" the Soviet Union, and so on. Such developments combined, it is said, not merely to dull Moscow's taste for detente but also to give it an urgent need to protect a troubled flank.

There is enough surface plausibility in this explanation at least to muddy waters -- until you stop to consider that the step under question here is not just another routine hardball move in the standard geopolitical game, but a most unroutine, indeed, unprecedented, move: an unprovoked invasion of a friendly sovereign state. We suspect the encirclement theory represents mostly a neurosis the Soviets find it convenient to peddle abroad. Regardless, it does not justify Moscow's act.

Then comes the tricky part of the Soviet explanation. The Politburo's doves, including Leonid Brezhnev, the political patron of detente, and Alexei Kosygin, the economic patron, resisted the invasion, it is being put about, but the aforementioned American failings had removed their high cards, and they have been sick. The chief hawk, party ideologist Mikhail Suslov, finally got the whip hand, or so the Soviet whispers were going before Mr. Brezhnev's blast at Mr. Carter yesterday.

Well, maybe so.We weren't under the table in the Kremlin and it is at least feasible that so bold a step as the Afghan operation would have been a close call. Except we aren't convinced. The explanation being retailed now leans in an all too familiar way on what you might call the used-car lot model of Soviet politics. There are always two salesmen: the bad guy who will never give you a fair deal on your trade-in -- for 30 years it's been Mikhail Suslov -- and the good guy to whom you supposedly turn in relief and gratitude to close the deal. The Kremlin's signal now is that the bad guys invaded Afghanistan but the good guys remain available if the United States still wants detente.

The used-car lot theory sounds so reasonable and appealing. Yet it overlooks that Leonid Brezhnev is not only the patron of detente but the author of the "Brezhnev doctrine," in whose name Moscow intervened to preserve the tender shoots of socialism in Afghanistan -- this much was evident even before Mr. Brezhnev spoke yesterday. And it does not explain how the United States can deal with the good guys without giving the bad guys exactly what they want. And, again, there is no real proof that the theory is true.

The United States can't know just why Moscow invaded. That being so, it would be silly to make American policy hostage to any one guess, least of all to an extreme guess: either that the invasion betrays an intent to start right now a relentless armed march toward world domination, in which case we should start preparing for World War III, or that it is a limited, unique and possibly accidental episode, in which case we should grimace and bear it. It is something, or more than one thing, in between: it is real, it is serious, it must be dealt with in a careful and calculated way, without illusion and without hysteria. Certainly, in figuring out how to respond, we cannot make policy as though we were buying a used car.