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.