THINK BACK a few months to the convulsions in Washington when the Walker spy case broke. What went wrong? What are the damages? How can a repetition be prevented? (And how is that review going, by the way?) The recollection allows a better appreciation -- a deeper relishing -- of what Soviet intelligence must be undergoing after the defection of the KGB chief in London. Having a few of your spies caught is one thing: breaks of the game. Having a spy, especially a rezident, defect opens up a whole new range of costs. True, had Oleg Gordievski stayed in place longer, the coup might have been more profitable. Establishing whether he is genuine can take an eternity. But it is nice for the American public to feel that its side wins one from time to time.

The British are expelling 25 other Soviets -- six diplomats, seven in trade missions, five journalists, and others, including the obligatory driver. Again, outsiders can't know whether some of them might have been more usefully left in place. The total, however, is eye-catching: 25 is not as high as the 105 Soviet agents lost in a single British sweep of 1971 or the 47 tossed out in a French sweep of 1983, but it's a lot of spies. The scale of the Soviet espionage effort is immense. It's hard to believe any Western country matches it.

Spying is essential and accepted: every security- conscious country must do it, and even its embarrassments become bureaucratized. This is why the British could explain that the mass expulsion is not expected to affect their relations with Moscow -- though the heavy-handed Soviet response in expelling arbitrarily 25 British citizens from Moscow surely calls into question Soviet professions of wanting good relations with London. The Soviets don't just do espionage; they overdo it. Just because they are more aggressive in intelligence-gathering is no reason to let them get away with it. It is good to see the British lowering the overall ceiling (by 23, to 211) of official Soviets in London; the subceiling for diplomats goes up (by 7 to 46). A rough rule of reciprocity ought to apply.

It's a tough rule to apply in the American-Soviet relationship. The Soviets bring their full staff to Washington; the Americans hire a good number of lower staff in Moscow, and the post-"star dust" effort to change the pattern will be slow in producing results. While embassy levels can be kept relatively even, however, there is no balancing the Soviet presence at the United Nations -- 260 in official missions, 300 in the secretariat, not to speak of the nationals of Soviet client states. Effective counterintelligence is probably the only answer.