Susan Rice, the U.S. ambassador to the United Nations, was seeking to describe what makes the Obama administration’s foreign policy distinct from that of its predecessors — not just the George W. Bush administration, but also the Democrats of the Bill Clinton years.

Her comments hinged on the Vietnam War. “We just don’t have that Vietnam hangover,” Rice told me in an interview last year. “It is not the framework for every decision — or any decision, for that matter. I’m sick and tired of reprising all of the traumas and the battles and the psychoses of the 1960s.”

With every president and administration, journalists and analysts embark on a quest to identify a doctrine or set of principles defining the group’s foreign policy. Are they realists? Internationalists? Neocons? Do they go it alone or lead from behind?

But to understand the Obama administration’s approach to the world, it helps to think in generational terms, not foreign policy slogans. Rice’s remarks highlight the twists and turns that the Democratic Party has taken over the past four decades, and how the interplay of three generations has shaped the Obama administration’s views on the use of force and America’s role in the world, as well as on specific challenges ranging from Afghanistan to China.

The first, eldest cadre of Democrats is the post-Vietnam generation: those foreign policy hands who started their careers in the 1960s, ’70s or ’80s. Next come the post-Cold War Democrats, who began working on foreign policy during Clinton’s administration. The third and youngest group, which I call the Obamians, is made up of post-Iraq war Democrats — the president and some of his closest aides, who did not become involved in the execution of U.S. foreign policy until 2009.

In conversations with members of all three groups, Vietnam is a recurring symbol. “The president’s conception of power is not founded on Vietnam. He’s the first president who’s not trying to justify himself in the context of that very tumultuous period,” asserted deputy national security adviser Denis McDonough, who has worked alongside Barack Obama since his first presidential campaign.

Obama is not the first Democratic leader to define himself as transcending Vietnam. At least since the 1980s, many of the party’s political candidates (think Clinton or Gary Hart) have portrayed themselves in that way. Yet in the somewhat self-serving logic of the Obamians, those earlier Democrats were still influenced by the war: They reacted against it by trying to prove that they were tough and willing to use force — that they were not like the antiwar Democrats of the Vietnam era.

In 2010, I asked a couple of Obama’s close aides about their party’s political vulnerability on national security. I had in mind the defeats of Democrats such as George McGovern and Michael Dukakis, whom Republicans portrayed as weak on defense. But the aides’ answer was surprising: “Oh yes, we call it the 2002 problem,” one of them said.

Why 2002? That was the year Democratic leaders in Congress voted to authorize Bush to use force in Iraq. The senior Democrats’ acquiescence became the Obamians’ formative foreign policy experience. In fact, in 2008 the Obama campaign attacked the more experienced Democrats of Hillary Rodham Clinton’s team by linking her to Bush’s unpopular war.

Indeed, Obama and some of his young aides can validly claim to be the first administration not affected by Vietnam. Obama is the first president in the modern era who neither served in the military nor was subject to the draft. His two immediate predecessors, Bush and Bill Clinton, were questioned during their campaigns about their draft record or military service. But Obama was 13when American troops came home, and several of his close aides were even younger. They took as a given the existence of the volunteer professional army; the military is to them a constituency, not an emotional tug.

The embodiment of the oldest generation, the initial post-Vietnam generation, was the late Richard Holbrooke, Obama’s special envoy for Afghanistan and Pakistan. One administration official told me that during discussions on Afghanistan, when Holbrooke talked about the lessons of Vietnam, others in the room sat there rolling their eyes. When he cooperated with a New Yorker magazine profile tracing his career from Vietnam to Afghanistan, McDonough called him to the White House and chewed him out. The comparison between Afghanistan and Vietnam was not one that Obama found helpful.

The administration has included several other members of the post-Vietnam generation, such as Vice President Biden, former White House counsel Greg Craig, former director of national intelligence Dennis Blair and former Middle East envoy George Mitchell.

During the Vietnam era, such men did not embrace the the antiwar left; most of them sought to counteract it. But in doing so, they struggled to cope with a widespread mistrust of American power and a sense of national decline. In one mid-1970sarticle in Foreign Policy magazine, Holbrooke denounced “the Vietnam-based, guilt-ridden anguish of the left” and debunked the idea “that because America has done some evil things, America itself is an evil force in the world.”

This generation took a variety of lessons from the war — above all, how an ill-considered, open-ended military intervention can lead to disaster. This constant reminder has not always been welcome among the Obamians, though, particularly when the team was deciding to send additional forces to Afghanistan or to dispatch warplanes over Libya. No surprise that the post-Vietnam generation has often been marginalized or isolated within the Obama team, with Biden a notable exception.

The post-Vietnam era came to a close in 1991 with America’s victory in the Persian Gulf War and the Soviet Union’s collapse. Soon a new generation of Democrats rose through the Clinton administration ranks at the State Department, the Pentagon and the National Security Council. They were, on the whole, more confident of American power and prosperity than the post-Vietnam Democrats. They felt little need to prove that the United States was a force for good in the world. The question preoccupying them was not whether the nation had the right and the power to send forces overseas, but whether and where this power should be used (Somalia? Bosnia? Haiti?).

After Clinton left the White House, these second-generation Democrats argued — in books, op-eds and study groups — that the party should recognize the continuing relevance of military power. “Force should never be used as a first choice, but in some cases it may need to be used sooner rather than later, particularly when innocent lives are at stake or when grave dangers are emerging,” wrote several prominent officials from the Clinton administration in a study group called the Phoenix Initiative.

The title of a book co-authored by Kurt Campbell, one of these second-generation Democrats, captured the spirit in two words: “Hard Power.”

When he came to the White House, Obama needed experienced people to fill foreign policy jobs, and the Clinton veterans were ready and waiting. Several returned to office under Obama, including Tom Donilon, now national security adviser; Antony Blinken, the vice president’s top foreign policy adviser; Michele Flournoy, Obama’s former undersecretary of defense; Campbell, the assistant secretary of state for East Asia; and James Steinberg, the former deputy secretary of state.

Over his years in office, Obama has evolved and now is running for reelection as something of a Hard Power Democrat, highlighting his prowess in the use of force. Still, generational differences persist between the Obamians and the Clinton alums. For example, Bill Clinton and his secretary of state Madeleine Albright spoke of America as the “indispensable nation.” As secretary of state under Obama, Hillary Clinton has offered similar themes. “The United States can, must and will lead in this new century,” she said in a 2010 speech.

But when Obama’s younger aides talk about America’s role in the world, there is a subtle recognition that its post-World War II dominance may not last forever. “We’re not trying to preside over America’s decline,” deputy national security adviser and Obama speechwriter Ben Rhodes observed in an interview. “What we’re trying to do is to get America another 50 years as leader.”

The distance between the Obamians and the post-Vietnam generation endures, too. In theory, the Vietnam experience is relevant to some of the problems the Obama administration confronts — for example, in negotiating with the Taliban while seeking to withdraw forces from Afghanistan.

But on the whole, the Obama Democrats don’t want to think about Vietnam. It was the preoccupation of an earlier generation, one that they see as having dominated American foreign policy for too long.

Rice recalled her exasperation when she worked for John Kerry’s presidential campaign. “What frustrated me about the 2004 campaign was, there we were, relitigating ‘Where were you in nineteen sixty-whatever?’ as the big freaking issue between Bush and Kerry — you know, ‘Did you serve, did you not serve, what did your swift boat brothers think?’ ” she said. “And I’m thinking, ‘What does that have to do with me and the world we’re living in today?’ ”

James Mann is author in residence at the Johns Hopkins School of Advanced International Studies. This article is adapted from his new book, “The Obamians: The Struggle Inside the White House to Redefine American Power.”

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