THE NATO we have known for 40 years may be suffering from a case of terminal success.Its enemies have become friends, leaving it with no borders to defend. States once dependent on the purse and power of the United States have long since grown prosperous, muscular, and independent. The organization's mission has receded to the point where it is little more than a vehicle for preserving US involvement in European affairs and a hedge against a resurgent, expansionary future Russia. If NATO did not already exist, few today would see the need for its invention. Were it to be designed from scratch, it would be different from the current organization in several respects: * Outdated military doctrines like the "forward defense" of eastern borders and a "flexible response," permitting the first use of nuclear weapons to repulse a conventional attack, would be discarded. * There would be a substantial "out of area" role, embracing crises in Eastern Europe. * The dominant power and influence, long wielded by the US, would be more equitably shared, as would economic burdens. * The size of the US force presence in Europe would number about 50,000 rather than the 150,000 now planned as a statement of enduring credibility. Even with such changes, the monopoly status in Western Europe enjoyed by NATO would likely come under increasing challenge. Absent such changes, NATO could find itself trampled by a West Europe galloping toward greater economic and political integration. That France, a key player in the West European Union (WEU), left NATO a generation ago decrying the organization's subordination to Washington, only emphasized the point. This is the context for recent Franco-German talk about the formation of a corps-sized unit outside NATO; in this context the corps will be discussed at the NATO summit this week. Much of the talk in Paris and Berlin about a future Euro-army of 100,000 is barely sketched out, with key questions about its mission, deployment area, and command structure left unanswered. Still, the response of NATO's old guard to the development showed it touched a tender nerve. Secretary General Manfred Woerner declared there was no place for a "rival force" on NATO soil. The British and Italians sought to channel the idea into a less divisive form, proposing the move of WEU headquarters to Brussels where, in close collaboration with NATO, a companion all-European military organization is likely to be expanded at the Rome summit. But the logic for a more ambitious all-European arrangement is compelling. The West Europeans are best suited to intervene as peacekeepers in Eastern Europe should some future conflict there threaten to produce ethnic slaughter, human chaos, or legions of refugees streaming west. Even should events not reach that pass, the discreet establishment of security relationships with former Warsaw Pact states now seeking protection from the West is a task which can be performed less provocatively by an all-European force. Moreover, with the Soviet Union in a state of disillusion, the domestic consensus is building behind new and deeper cuts in the defense budget. While 150,000 US forces in Europe might well be what Pentagon leaders describe as "a prudent way-station" in the midst of dynamic change, they may soon have to stop thinking about a presence large enough to be "credible" and begin contemplating instead one small enough to be sustainable. Rather than dismissing a "symbolic" US presence in Europe as a political defeat, they might instead consider it for what it is: final victory.