NEWS ANALYSIS : Clinton Faces Leadership Challenge on European Trip : Diplomacy: He hopes to establish his credibility among allies and at home. His foreign policy gaffes and Vietnam War position pose obstacles.

As President Clinton prepares to depart for Europe this week to commemorate the 50th anniversary of D-day, he faces the challenge of asserting America’s--and his own--leadership in the world.

Speaking across a generational chasm and addressing himself to veterans of a military whose later mission in Vietnam he and others vehemently opposed, Clinton confronts an important test of his presidency.

Can the 47-year-old Clinton convince a skeptical world that he and his fellow postwar baby boomers are ready to assume the mantle of leadership? Can he summon a nation and its allies to deal with a world whose threats are far more diffuse and ambivalent than the unvarnished evils of Nazism and fascism?

And can he present a coherent view of American engagement in the world despite a foreign policy record marked by vacillation on Bosnia, retreat in Somalia and Haiti and reversal on human rights in China?

The way the world and the American public come to answer those questions will have a lasting effect on how much Clinton will accomplish in his presidency. Clinton hopes that by proving himself a strong spokesman for America’s ideals abroad, he can boost his credibility at home despite questions about his character and competence.

Aides say the key themes of the European trip will be homage to the heroes of the “Great Crusade” of World War II and a vision of the future based on the triumph of democracy.

As Clinton speaks against the backdrop of American military cemeteries in Italy, Britain and France, he and millions of television viewers will be reminded of the painful legacy of the bloodiest century in human history.

Clinton, who was born after the war ended, will be shadowed not only by his own draft-avoiding past but also by the specter of former President Ronald Reagan.

Reagan, who spent World War II in Hollywood making training and propaganda films for the Army, staged what many consider the most effective piece of political theater of his presidency on the cliffs above the Normandy beaches during the 1984 ceremonies celebrating the 40th anniversary of the D-day landings.

White House aides say they know that comparisons with Reagan’s stirring performance are inevitable, but they contend they are irrelevant. In 1984, they say, the United States was still engaged in a great moral struggle against communism and Reagan’s words about the heroes of Normandy carried a resonance that cannot be reproduced today.

Clinton will instead try to summon up the valor of the struggle against fascism and make it apply to the effort to consolidate democracy in the nations of the former Soviet empire and elsewhere around the globe.

Aides involved in planning the trip and polishing the keynote speeches said there will be no chest-thumping over the Allied military triumph of World War II.

“Let me emphasize this: This should not be seen as a victory over Germany and over Italy,” said National Security Adviser Anthony Lake. “I think the President will be trying to make it clear that we are not celebrating the defeat of certain nations; we are celebrating the victory of an idea, a liberating idea, of democracy.”

Lake said Clinton intends to honor not only the veterans of the war but also those political and military leaders who shaped the postwar world, rebuilt the shattered economies of Europe and Japan and led the West in its costly victory in the Cold War.

With fascism and communism vanquished, Western leaders must now find another organizing idea, Lake said, and that will be a recurring theme on Clinton’s trip.

“Our challenge today, unlike the generation of World War II, and really unlike the generations that prevailed during the Cold War, is to somehow find the same unity of purpose, the same spirit in time of peace to accomplish two very important tasks,” Lake said.

The first is the political and economic renewal of America; the second is continued American engagement on the world stage, sometimes acting alone, at other times in concert with the United Nations or other alliances.

In addition to marking the anniversary of the liberation of Europe, Clinton will meet with French and British leaders as well as officials of the new government of Italy.

The discussions will touch on economic issues and on the continuing disputes over Bosnia and the future membership of Russia and other former Soviet states in the North Atlantic Treaty Organization.

In preparation for the trip, Clinton has been studying World War II history and meeting with staff advisers and outside experts. On Wednesday, he hosted a dinner for historians and D-day veterans to get a feel for the momentous event.

Stephen E. Ambrose, whose recent book on D-day has been Clinton’s bedside reading for the last two weeks, was among those who attended the dinner, which was preceded by a formal briefing from military historians in the White House theater.

“He had a copy of my book and it was well-thumbed and all marked up,” Ambrose said. “He’s taking this with great seriousness and asked good questions. He wanted to know how they kept the secrets so well. He wanted to know what World War II would have been like if CNN had been there.

“He recognizes he has a great opportunity here. Unstated was that there is some risk (for him personally). There’s a delicacy that went unspoken,” Ambrose said.

Another participant in last week’s dinner, Brig. Gen. Harold W. Nelson, the Army’s chief of military history, said the subject of Clinton’s own avoidance of military service during the Vietnam War didn’t come up.

But Nelson, who served as an adviser to the South Vietnamese army in 1965, said Clinton should have little trouble relating to World War II veterans at the observances at Normandy and elsewhere.

“I don’t think that’s a problem,” he said. “Franklin Roosevelt didn’t serve in uniform; our wartime presidents have not had to prove themselves by military service.”