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Ken Burns and Lynn Novick's comprehensive documentary series on the Vietnam War is filled with the stories and voices of ordinary soldiers on all sides of the conflict. But the most tragic aspect of the tale, for me, was hearing President Lyndon B. Johnson on tape, before full U.S. engagement, admitting that the war could not be won. Johnson's dilemma is one that presidents dread facing — and one that President Trump is bringing upon himself with North Korea and Iran.

In May 1964, when the United States had fewer than 20,000 troops in Vietnam, serving as advisers and trainers, Johnson said to his national security adviser, McGeorge Bundy, "I just stayed awake last night thinking about this thing. . . . It just worries the hell out of me. I don't see what we can ever hope to get out of there with once we're committed. . . . I don't think that we can fight them 10,000 miles away from home and ever get anywhere in that area. I don't think it's worth fighting for, and I don't think we can get out."

"I look at this sergeant of mine this morning," Johnson continued. "He's got six little old kids . . . What in the hell am I ordering him out there for? What the hell is Vietnam worth to me? . . . What is it worth to this country?"

Johnson was asking all the right questions. He understood that Vietnam was not actually vital and that it could easily become a quagmire. And yet, he could never bring himself to the logical conclusion — withdrawal. Like so many presidents before and after him, he could not see how he could admit failure. No president could do that. In another conversation, with his mentor from the Senate, Richard Russell, Johnson speculated that "they'd impeach a president, though, that'd run out [of Vietnam], wouldn't they?"

And so, because the president of the United States could not think of a way to admit that the United States needed to reverse course, Johnson increased troop levels in Vietnam from fewer than 20,000 to more than 500,000, tearing apart Indochina, American society and his presidency. The example is dramatic, but it is generally true that in foreign policy, when the United States is confronted with a choice between backing down and doubling down, it follows the latter course.

In two crucial arenas, North Korea and Iran, Trump has dramatically raised the risks for the United States, and for no good reason. Determined to seem tougher than his predecessor, he has set out maximalist positions on both countries. He wants a totally denuclearized North Korea and an Iran that stops making ballistic missiles and stops supporting proxy forces in countries such as Syria, Iraq and Yemen. There is a vanishingly small possibility that North Korea and Iran will simply capitulate because Washington demands it. And if they don't, what will Trump do? Will he back down or double down? And where will this escalation end?

Trump seems to view international negotiations as he does business deals. He has to win. But there is one big difference. In the international arena, the other person also has to worry about domestic politics. He or she cannot appear to lose either.

As a leading businessperson recently said to me, "Trump is playing a two-person negotiation, thinking it's just him and the other guy, two principals, making a deal, as in business. But actually there are people outside the room — the two nations' publics — that place huge constraints on the negotiators. It's not a two-person game at all."

For any international negotiation to succeed, there has to be some element of "win-win." Otherwise, the other side simply will not be able to sell the deal back home. But Trump seems to believe above all that he must win and the other side must lose.

A senior Mexican official told me that there would have been a way to renegotiate the North American Free Trade Agreement, even find a way to fund the border wall, "but Trump needed to allow us to also declare some kind of victory, give us some concessions. Instead he started out by humiliating us and made it impossible for [President Enrique] Peña Nieto to make a deal. After all, no Mexican government can be seen to simply surrender to Washington."

Trump's way of negotiating might have worked in his past life, although there, too, many argue it was not the way to build a great reputation. But he's not doing real-estate deals anymore. The arena is different, the conditions are far more complex, and the stakes are higher — astronomically higher.

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