THE United States is facing an unusual problem: We need to reassess a policy that has been a resounding success. Providing for the security of Western Europe through the North Atlantic Treaty Organization (NATO) has worked so well that now we have to ask ourselves, what do we do next?It will help to review how the present situation evolved. In 1949, when NATO came into being, the US had a monopoly on nuclear weapons, but the Soviet Union was superior to the West in almost every category of ground forces. Public opinion, especially in the US, was anti-communist, but that did not extend to supporting large, standing military establishments. At no time did the members of NATO meet the conventional-force goals that their defense and foreign ministers accepted in the North Atlantic counci l. But no matter: The American nuclear umbrella protected Europe long after the American nuclear monopoly ended. But there have been two changes. The first is the collapse of the Soviet Union and its empire in Eastern Europe. The Warsaw Pact, a counterweight to NATO, no longer exists. Soviet troops are going back to the Soviet Union. Nowhere in Eastern Europe, and especially not in the Soviet Union, can the economy support a major military effort. The second change was brought about by the startling performance of American non-nuclear forces in the Persian Gulf war. For whatever reasons, the Iraqis chose not to put up much of a fight in that war, but such fighting as they did was with weapons that were top-of-the-line in the Soviet inventory. The contrast between our weapons and their weapons was stark. The conclusion is that nuclear weapons are no longer as important as they once were to offset Soviet numbers. But for the Soviet Union, they may be more important; they may now be seen as an offset to superior US non-nuclear technology. Once the US lost its nuclear monopoly, the American protective nuclear umbrella over Western Europe became part of a standoff between the two superpowers. The standoff worked because there was stability, and there was stability because there was a standoff. Now, there is only one superpower; there is no more standoff; and more unnerving, there is no more stability. For 40 years or more, the US spent nobody knows how much money on various schemes, some of them rather harebrained, to destabilize the Soviet Union. Now it is destabilized and we are not sure what to do about it. The instability introduces a new factor in the nuclear equation: Soviet nuclear missiles are still in place, still presumably programmed to hit targets in the US and Western Europe. Who controls the firing mechanisms of those missiles is crucially important to the US. THIS makes a US-Soviet nuclear arms control treaty more urgent than it was in the days before the political uncertainties attendant on Soviet internal turmoil. It probably also complicates the Soviet negotiating posture, because the Soviet military establishment will be more reluctant to give up what it may regard as its ace in the hole. Meanwhile, the nuclear-proliferation genie is long since out of the bottle, and this complicates the US negotiating posture. While we are talking with the Soviets, we have to keep one eye on other nuclear powers or putative nuclear powers. It would be nice if the US and the Soviet Union could mutually destroy all their nuclear weapons, but most Americans would be uncomfortable in a world in which Libya, Israel, Iraq, Pakistan, and India had nuclear weapons and we did not. NATO does not have much to do with any of this. The end of the cold war has set NATO bureaucrats scrambling to find a new mission. It is argued that NATO is a way to keep the US involved in Europe, but even without NATO, the US could scarcely extricate itself from Europe - there is too much American investment in the Economic Community. A better organization than NATO to deal with European security in the current context is the Conference on Security and Cooperation in Europe (CSCE). CSCE includes both West and East, and a part of Western policy ought to be to integrate the Eastern countries, including the Soviet Union, in Europe. That in itself would increase security. Europe played a valuable and commendable role in the Persian Gulf war, but NATO did very little. Its most powerful member - Germany - sat out the war, pleading constitutional disability. Disbanding NATO doesn't mean abandoning Europe. The US has too much at stake in Europe for that. Disbanding NATO simply means adjusting to new realities.