A new chapter is opening in transatlantic relations as NATO formally admits its first three former Soviet-bloc foes, bringing more of Europe under the American-led security umbrella.
Bolstered by its cold-war triumph, a Bosnia peace-keeping mission, and a vision of a united and democratic Europe, the pact expands to 19 nations today with the induction of Poland, Hungary, and the Czech Republic.
Yet the ceremony at the Harry S. Truman Library in Independence, Mo. - a tribute to the man who oversaw NATO's creation as a bulwark against a Soviet thrust into Western Europe - masks deep apprehensions over the alliance's future.
Driving these concerns are disputes over the Kosovo crisis, which is testing NATO's credibility and cohesion, and huge gaps between American defense spending and capabilities and those of the allies. Some countries, especially the new members, will take years to catch up.
There are also differences over new missions, future threats, and nuclear policy. Added to that mix are European ire at unilateral American policymaking, the United States' frustration with Europe's new political assertiveness but reluctance to use force, and transatlantic feuds on trade, Iraq, Iran, and Cuba.
All this doesn't mean NATO is doomed. But a decade after the fall of the Berlin Wall, history's most successful military pact still is struggling to find its footing in the absence of a Soviet-style foe.
"The alliance is still solid, but not like it was," says Stanley Sloan, a European security expert at the Congressional Research Service. "These disagreements ... can have a greater effect on the overall relationship."
Agrees Ivo Daalder, a former National Security Council official now at the Brookings Institution: "We are clearly in an evolutionary mode in trying to figure out how the transatlantic relationship is going to change."
NATO expansion is a major accomplishment for President Clinton, its chief sponsor. Despite popular reticence over new overseas entanglements and greater concern with domestic issues, the plan he unveiled in 1995 commands overwhelming support on Capitol Hill and the approval of 64 percent of Americans.
But NATO expansion is only part of a wider initiative Mr. Clinton has pursued to solidify Europe and avert feuds like those that ignited two wars in which tens of thousands of Americans died. Given the contentious issues facing the pact, he seems certain to leave office before his goal is realized, leaving his foreign-policy legacy incomplete.
"The one area of his foreign policy where Clinton has a very clear vision is Europe," says Mr.
Daalder. "He has put in place a strategy ... and it is not clear if that strategy will survive him."
Indeed, transatlantic differences are forcing the administration to scale back its agenda for NATO's 50th-anniversary celebration here next month. US officials and other experts say the White House is compromising on key issues to avoid spoiling the largest summit of international leaders the capital has ever hosted.
Concedes a US official: "This is clearly something that involves some choreography."
The US had promoted the meeting as the jewel in a "triple crown" of three summits this year that would cement transatlantic ties for the next century. The security relationship is to be set in a post-cold-war blueprint for NATO's future known as the New Strategic Concept.
But as US and allied officials began drafting the document, differences over key issues arose. Chief among them is that of new missions that do not involve NATO's central pillar of mutual defense. While all members agree they must be involved in the strife-fraught Balkans, many oppose the US proposal to involve NATO in threats from outside Europe, especially the Middle East.
"The [US] aim was rather high with respect to a NATO that defends common interests," says the US official. "Most of the countries in Europe don't want to be dragged into something they are uncomfortable with."
Unable to resolve this question, the summit is expected to paper it over - and weigh future missions on a case-by-case basis. The same approach is likely on the issue of undertaking "out-of-area" missions without a United Nations mandate - as the US and Britain urge - or only with one, as France and other allies contend.
UNLIKELY to be considered at all is the issue of nuclear policy. Canada, Germany, and several other members want to consider a "no first use" declaration. But the Pentagon opposes changing the current doctrine of deliberate ambiguity, which embraces nuclear weapons if needed to repulse conventional attacks.
While the summit may skirt potentially disruptive issues, a full-scale war in Kosovo could force the alliance to confront some of them. NATO has pledged to use force to avert war in the Yugoslav province. But members are divided on making good on that threat, even as a US peace plan seems closer to collapse.
If war erupts between Serbian forces and ethnic Albanian rebels, NATO's credibility - and US leadership - will be damaged, many analysts say, intensifying doubts about its future.