From the beginning, John F. Kerry has formed his presidential campaign around his service in the Vietnam War. But it is another war, World War II, that shaped his belief that the United States is more effective on the global stage when it works closely with other countries.

As a young boy, he saw the ruins of his mother's house in France. As an adolescent, he lived in West Berlin, where his father was a diplomat. Even as a young man, he measured his service in Vietnam against what his experience and education had taught him about the importance of international alliances.

Now Kerry's belief in the need for global partners has become one of the defining elements of his challenge to President Bush. In tonight's debate, the Democratic nominee is expected to stick to the theme he has sounded for months: Only a change in U.S. leadership will attract other countries to share the burden in Iraq.

That line of attack has not been without hazard. Bush and Vice President Cheney have used it as a basis for counterattacks, with the president portraying Kerry as wanting to submit U.S. policy to a "global test."

The practicality of Kerry's approach is itself a matter of debate. If he wins, Democratic and Republican analysts said, Kerry might enjoy a honeymoon and get more support from allies on "soft issues," such as debt relief and training Iraq's army and police. But they said additional foreign troops will be hard to come by as long as the insurgency rages.

"There is no magic bullet here. If Kerry comes into office, the mere fact that he did not start this war is not going to create a miraculous new beginning with our allies. It's only modestly helpful," said Larry Diamond, who worked with the U.S. occupation government in Iraq. "What leaders are going to want to face their publics as the violence continues and say, 'We need to put troops on the ground' "?

Still, Kerry's devotion to alliances forms the core of his worldview, longtime friends and associates say, an outlook shaped as much by biography as by ideology. His handlers and his detractors have focused on his service in Vietnam, but a more essential chapter in his life is his youth in postwar Europe.

In 1947, not long after the war ended, Kerry's mother took him to her childhood home in the Brittany region of France. Rosemary Kerry, who had moved to the United States as a young woman, told her son that she had fled on a bicycle, ducking machine-gun fire from German bombers.

Kerry, then 4, held his mother's hand as their feet crunched over stones and shards of glass. The walls were heaped at their feet. A chimney rose above the rubble; a blackened stairway broke off at the sky.

The Nazis had used her house as a military post. Before they retreated, they blew it up, Kerry said in an interview, because his grandparents had been friendly with the British prime minister, Winston Churchill.

"It's my earliest memory," Kerry said, sitting in his study in his Boston home. "She was sobbing; I was perplexed." He reached for a photograph of the Brittany house and traced the black-and-white lines: "Here's the chimney. The grass was tall."

Kerry keeps a second old photograph in his home. It is of his father, in a white U.S. Army Air Corps uniform, smiling beside a warplane. Richard Kerry volunteered before the Japanese struck Pearl Harbor.

These two images of war -- of mother and of father -- were etched in the boy's consciousness. The Allied forces had liberated his mother's home, and the Allies' cause had inspired his father's conscription. Evil, Kerry has said he learned, was defeated by an international alliance.

He cites a second, visceral experience as cementing his faith in international ties. Kerry was 11 when his family moved from Washington to Berlin -- old enough to understand that the bombed-out buildings were from World War II, young enough to pretend that they were still smoldering.

It was 1954, and Kerry was living in the hottest spot of the Cold War. His father, then a State Department lawyer, had become a legal adviser to James B. Conant, who supervised West Germany's rehabilitation. A family outing on a Berlin lake ended in panic when their sailboat drifted toward the Russian-patrolled shore.

"It was a triggering period of my life," Kerry said in his Senate office, sitting in front of a World War II bond poster. "I became profoundly intrigued by global confrontation." Kerry saw refugees and signs reserving seats on buses for "mutiles de guerre" -- war wounded.

On the other hand, when he walked into a hotel, the wall plaque made him proud: "Built With Aid From the Marshall Plan." The Americans and their allies were rebuilding Europe and protecting its people from the threat of Stalinism.

"War was a mix," said Kerry's sister, Diana. "I hate to add to the nuances that characterize John, but it wasn't clear-cut that war was bad. We saw places in ruins, but on the other hand there was this sense of heroism."

Kerry's brother, Cam, summarized their parents' thinking: "War is noble hell. It can be a noble enterprise, but has terrible costs. Treat it wisely."

For Richard Kerry, that meant creating alliances. He was one of the cutting-edge pro-NATO U.S. diplomats, dedicated to transatlantic ties. The elder Kerry introduced his son to the architects of modern Europe, including Jean Monnet of France, a forefather of the European Union.

But it was while riding his bicycle that John Kerry absorbed one of his most enduring lessons. One day, he rode into Berlin's forbidden eastern sector. While campaigning in Iowa last year, Kerry recounted the adventure: "I biked around Hitler's bunker -- a huge slab of concrete blown up." Seeing proof of Hitler's defeat by the Allies, he said, "taught me the value of trying to build coalitions."

The lesson remains in place. "Coalitions are what he's all about," said Rand Beers, Kerry's national security coordinator. "You go back to World War II, to the role that NATO and allies played in the Cold War. His constant refrain is about international coalitions."

As a youth, Kerry admired generals who leveraged war into global cooperation. His military heroes were George C. Marshall, author of the Marshall Plan, and Dwight D. Eisenhower, who helped create NATO.

In college, recalled John Shattuck, Kerry's debate-team partner, Kerry defended a liberal foreign policy. "We defined liberal foreign policy as strong on defense, with its roots in World War II," said Shattuck. "You compete with military power, but also with ideas and alliances."

In Vietnam, Kerry's combat experience hardened his preference for multilateral over unilateral action. "Vietnam would have been okay to John Kerry if NATO was behind us," said Douglas Brinkley, author of "Tour of Duty," a Kerry biography. "He was not opposing Vietnam, he was opposing 'go it alone' intervention."

One factor influencing him, Brinkley said, was Richard Kerry, who protested the war because "we had no European allies. In 1966, France pulled out of the NATO integrated military command because of U.S. unilateral action in Vietnam."

After Richard Kerry retired, he published a book in 1990 that condemned the American establishment for neglecting international institutions. A former Kerry Senate aide said that in 1991, while Kerry was deliberating over whether to vote for the use of force in the Persian Gulf War, "John was concerned that diplomacy was not being pursued strongly enough. His father kept faxing him stuff from his book."

Today, Kerry concedes the limits of what he can do in Iraq. Bush's obstinacy, he said this week in Iowa, has complicated the effort to enlist other countries. He said he could do far better but cautioned that does not mean he can attract substantial new military help: "What I will do is bring new credibility, a fresh start, a presidency with the trust that will be able to bring allies to the table."

It is a theme he has beaten like a military drummer throughout his campaign.

"What he realized as a child was the primacy of Europe in America's foreign policy," Brinkley said. "You don't get swept away in sideshows like Vietnam if it's going to damage NATO. Essentially, it's what he's saying today about Iraq." Asked how Kerry's principles of war have changed over time, Brinkley said:

"I don't think he has changed since he was a boy."

Staff writer Dan Balz, traveling with Kerry in Iowa, contributed to this report.

John F. Kerry has said his father, Richard, shown here as a member of the Army Air Corps in World War II, taught him to have an international approach to world problems.