Last week, President Clinton journeyed to Brussels to assure some Eastern European countries that the United States will in due time furnish them protection from Russia. He proceeded to Kiev to seal an accord designed to prevent another Eastern European country from protecting itself.

Clinton's trip may not have achieved anything else worthwhile, but it did provide blinding proof that his policy on European security is exactly backward.

On the one hand, he wants to start integrating Poland, Hungary, Slovakia and the Czech Republic into NATO-a measure against Russian aggression that conspicuously lacks the military forces needed to make it believable. On the other, he would neutralize an exceptionally useful check on Russian imperialism-a nuclear-armed nation of 52 million, Ukraine, that sits astride the approaches to Europe. In truth, Eastern Europe would be far safer behind Ukraine's atomic arsenal than America's meager commitment.

Both of the administration's mistakes flow from the same source: an excessively sunny view of our former enemy. If from now on we can expect Russia to be as peaceable as Switzerland, there's no risk in pledging to defend Poland, since we'll never have to back it up anyway. And Ukrainians can sleep soundly without nuclear weapons because they have nothing to fear from the Russians.

But what if the Russians eventually start behaving not like Swiss but like Russians? Their history of treating neighbors like galley slaves didn't begin with communism and probably didn't end with it. In that case, one of the first targets will be Ukraine, which was joined to Russia for three centuries and which many Russians continue to regard as rightfully theirs.

Without the bomb, Ukraine will find it hard to resist being dominated by Moscow-just as it has in the past. That is doubtless one of the reasons many Russians enthusiastically endorse a nuclear-free Ukraine.

Ukrainians are not exactly oblivious to their predicament, which is why the de-nuclearization agreement with the U.S. and Russia shouldn't be taken too seriously. The parliament in Kiev, which claims the power to approve the decision, has plenty of members who oppose it. The elections scheduled for March may install more.

In any case, President Leonid Kravchuk, who may or may not be willing to forfeit his nuclear munitions, gave himself a leisurely span of seven years in which to carry out the promise-or to renege on it. By turning over some missiles now, Ukraine gets aid for its shipwreck of an economy and loses nothing important. Plenty of other warheads will be held back for the time being, and maybe for good.

Staying nuclear is the best guarantee of Ukraine's safety and independence-certainly much better than the squishy assurances that go with NATO's "Partnership for Peace," which Clinton said Ukraine is welcome to join. In a crunch, the Russians would have no grounds to think the U.S. and its allies would shed blood to defend Ukraine. But the Russians can be quite sure that Ukraine would defend itself with nuclear weapons, if necessary.

Kiev's nuclear arsenal also works to the advantage of Eastern Europe. Ukraine, which has every interest in discouraging Russian aggression, would be the natural ally of all the other countries worried about Moscow's intentions-starting with the four that want to join NATO. A Poland standing alone would be vulnerable to Russian extortion, but not a Poland standing shoulder to shoulder with a nuclear Ukraine.

By trying to strip Ukraine of its great equalizer, Clinton is preventing the kind of new regional arrangements that are needed to foster peace and stability in the old East bloc. In their place, he offers the hope that NATO will expand eastward, though not until such time as it won't offend the Russians. He wants-sort of-to do for Eastern Europe what it could do for itself.

What he can't alter, though, is the plain fact that the biggest force in NATO, the U.S., has no vital interests in Eastern Europe and therefore can't be relied on to protect it. Ukraine, which has vital interests in Eastern Europe because it is part of Eastern Europe, can't afford to stand aside if Russia reverts to its old habits.

Clinton is basing his entire policy on the fond wish that Russia won't regress. But the administration is not so optimistic that it's ready to give up America's own nuclear arsenal.

Ukraine would be smart to take similar precautions rather than rely on the benevolence of Russia or the fortitude of the U.S. And the U.S. should stop trying to deprive Eastern Europe of its best insurance against a future that may not be as tranquil as we hope.