Towards the end of World War II, the Vietnamese revolutionary Ho Chi Minh was still an obscure figure in his own country—past fifty, sick from malaria, and holed up in a complex of caves on the Chinese border with a couple of hundred partisans. He had never been to Hanoi, Vietnam’s capital, and to the extent that he was an asset, it was to the American General Claire L. Chennault. The first time the two met, Ho had walked for two weeks to Chennault’s Chinese headquarters, and he emerged from their meeting with a signed 8 X 10 glossy of the American officer. Soon he had more tangible evidence of collaboration: American military advisors attached to a unit called Deer Team, who helped to build the nub of the Vietminh army. Ho’s American handlers were anti-colonialists, and they believed in the Vietnamese cause. Ho had “the brightest eyes I ever saw,” said Charles Fenn, his first American contact. “I felt the wing of genius touch mine.”

The alliance was short-lived, and perhaps doomed from the start. But for a moment it was fervent. The Americans and the Vietminh shared a common righteousness and a common enemy. Ho’s unresolvable mix of communism and nationalism seemed, in those wartime days, a useful machine for killing fascists, and Roosevelt’s zeal for banishing colonialism permeated the American bureaucracy. During World War II, the French colonial government, allied with the fascist Vichy regime, had turned Indochina over to the Japanese; and the plunder that followed helped deepen a national famine, in which hundreds of thousands died. One OSS officer, observing French colonialism at work in rural Vietnam, wrote disgustedly of the “semi-slavery of the plantation coolies.” In China, the U.S. General Albert Wedemeyer stared down Admiral Lord Mountbatten, then the head of the South East Asia Command, and told him that “there would not be a British Empire after the war.” In Vietnam, Roosevelt issued a blanket prohibition on Americans working with the French, and, in the fall of 1945, the members of Deer Team marched on Hanoi with Ho, trudging through muddy deltas, picking green leeches off their skin, fighting alongside him in skirmishes. Two weeks after they left him, Ho arrived in Hanoi triumphant. His long military struggle to expel the French had really begun.

This struggle—which historians call the First Indochinese War—ended in 1954, when the French hunkered down in the Dien Bien Phu Valley, with the Vietminh General Vo Nguyen Giap’s cannon ringing the walls around them. This long and successful siege split Vietnam into a nationalist South and a communist North, and chased the French out of Indochina. By now, the Americans were on the other side of things entirely, paying 90 percent of French military costs. In France, public opinion polls put support for the war around 15 percent: even colonialism’s dead-enders were about to give up. And yet the Americans, who had a decade earlier been a force for liberation, were so bent on defending Dien Bien Phu that it took the intervention of President Eisenhower to overrule a proposal from the National Security Council to use tactical nuclear weapons against Ho’s armies, maddeningly obscured in the jungles and mountain tunnels. “You boys must be crazy,” Eisenhower replied. “My God!”

We often treat the Americans in Vietnam as an eruption of this kind of Strangelove-ism. The historical truth is more complicated. In 1952, John F. Kennedy, then a Congressman, visited Vietnam and came away clear-eyed: he warned that the Eisenhower administration was deepening its commitment to a crumbling and backwards politics. "In Indochina we have allied ourselves to the desperate effort of a French regime to hang on to the remnants of empire,” Kennedy said. Less than a decade later, Kennedy would make a set of similar mistakes. Vietnam ended as McNamara’s project, and his tragedy. But it began with the idealistic hope of Roosevelt, and Deer Team, and Kennedy, that third world liberation could be joined with anti-communism. Our adventure in Vietnam was also a tragedy of liberal belief.

Ted Morgan’s new book about Dien Bien Phu documents the moment when the politics of Vietnam pivoted, and the liberal faith that anti-colonialism and anti-communism might be melded began to fray. Morgan tells two stories, side by side. The first is an account—in increments of hours—of the slow, forsaken defense of Dien Bien Phu by the French troops. By French I mean, mostly, not French. Paris had refused to send domestic conscripts, because of the Indochinese War’s political toxicity (its costs swallowed up all of the country’s Marshall Plan money, postponing economic recovery), and so many of those in the trenches were Thai part-timers, or German hired guns. The most effective troops turned out to be Moroccans, who charged the communist lines crying Insh’allah!

Morgan’s French are equal parts valorous and dithering. (At one urgent moment, Washington pays for an airlift of reinforcements from Algeria, only to have the fresh regiments sit on the tarmac for days to preserve the integrity of the pilots’ vacation schedules.) But mostly they are doomed. The French, whose bias towards dying in forts had been preserved since the Franco-Prussian war, wanted a pitched battle, seen from embankments, rather than the village-scale skirmishes that they kept losing. The Vietminh wanted to draw the French away from their supply lines on the coast. The compromise was the long siege of Dien Bien Phu. The Vietnamese have never released casualty numbers from the battle, and most historians suspect that they were vastly higher than those the French sustained. But the Vietnamese kept coming, and, throughout the spring, the French base filled up with its own wounded. Evacuation was impossible.

At the end the French leadership turned abstract, debating among themselves the increments of valor inherent in various forms of surrender. There was an international conference at Geneva that would take up the question of Indochina, and Paris’s hope was that the garrison might hold out long enough to win more favorable terms there. They did not. “[I]t must not end with a white flag,” the leadership in Hanoi reminded General Castries, in command at Dien Bien Phu. So it ended without one, the French ceasing their firing and the Vietminh “swarming into the heart of the base.” Morgan observes, “The end of a battle is as disorderly as a battle itself, but less noisy.”

So what happened to the Americans in this story? How did a young and righteous nation come to back a tired set of values and an exhausted band of troops? This is Morgan’s second story. A part of the answer is simply that Roosevelt died. Truman had little of the same sense of common cause with the world’s oppressed, and when the military complained that the refusal to support the French in Indochina was “a source of serious embarrassment” to them and complicated operations elsewhere, he quickly acceded. Eisenhower’s foreign policy was largely designed by Secretary of State John Foster Dulles (“the only case I know of a bull who carries his china closet with him,” noted Churchill) who had such an evangelical notion of America’s place in the world that he often failed to notice the world itself.

But this was not just a matter of individual character. The Americans, for all their flaws, saw the shape of Vietnam’s politics clearly. The bureaucratic record in Vietnam is a decade’s long memo-chain describing the vile practices of the French, and the incompetence and unpopularity of the Vietnamese anti-communists who followed them. Even the hardest Cold War hawks, in the 1950s, were clear-eyed. Barry Goldwater, of all people, declared that “[b]y supporting France, we are saying to the great men who penned the document and whose ghosts must haunt these walls, that we do not believe entirely in the Declaration of Independence.”

The compromise that seduced the Americans was the French commitment to grant some measure of home rule to the government of the Vietnamese playboy Emperor Bao Dai. The Americans wanted to believe in someone like Bao Dai, for both ideological and practical reasons. There had been two camps within the State Department from the beginning. The Far East experts argued that the French were inextricably unpopular in Vietnam, and that the United States should not intervene to protect a dying and reprehensible regime. The Europeanists, meanwhile, argued that support for a shady colonial initiative in Indochina was a small price to pay for French assistance in Europe, and particularly for Paris’s participation in the anti-communist military league that would become NATO. Bao Dai tipped the balance, and the Eisenhower administration began to supply the French military. Soon, this would be built up into a kind of zeal. “We must demonstrate to Congress that the things the French are doing are important to the free world,” Dulles said.

Morgan believes that the Bao Dai solution, and by extension the notion that South Vietnam was not artificial but a real and defensible entity, was an obvious disaster. The French project to train a native army quickly collapsed, and Bao Dai himself was busy buying villas in Morocco, and a private plane. But despite Bao Dai’s obvious limitations, you could, squinting, make out the rough contours of a state forming around him and around his successor, the ultimately doomed President Ngo Dinh Diem.

Several large groups that had fought the French—Buddhists, Catholics, but also a set of heterodox religious groups—switched sides to support Bao Dai and to fight the communists. The new state had American money, and the earnest promise of help in development and governance. And there was the lingering fear that Ho Chi Minh’s communism was a stalking horse for Vietnam’s ancient abusers, the Chinese. The Americans might have been naïve, and altogether hopeful, when they gazed into this mess of conflicting interests and saw the possibility of a state. But they were not what they would later become in southeast Asia: they were not obsessed, or absurd.

There is a gaping hole at the core of Morgan’s work, and it keeps him from really approaching some of the most basic questions of this history. The problem is his sources. Morgan has an ear for the vanities and elegance of high diplomacy, and he sketches the senior Western figures with wit and grace. But I scoured the book looking for Vietnamese input, and only one appears frequently—the memoir of the Vietminh General Vo Nguyen Giap, which had already been translated and widely used by Western historians. Vietnamese politics and society do not make a real appearance in a book that is about the collapse of a system of governing Vietnam. Here is Morgan’s description of Vietnamese rural culture: “The iconic scene: a peasant wearing his conical hat, bent over a wooden plough pulled by a waterbuffalo.” This is a cartoon. At this late date in the study of this subject, it won’t do.

This limits Morgan—known for his biographies of Roosevelt, Churchill, and W. Somerset Maugham—to a blinkered treatment of the history, in which a few men thirty years out of Hotchkiss argue about a static world. But was the French loss at Dien Bien Phu inevitable? Could the South Vietnamese army have worked, if more resources were directed that way? How ambivalent, at this early stage, were Vietnamese peasants about Ho’s communism? How hopeful was the dream of a free and independent South Vietnam? Contemporary historians have begun to probe the Vietnamese archives and the pictures these scholars have drawn, though mixed in their conclusions, have filled out a more complex picture of Vietnam and of South Vietnam in particular. The American dream—that the Vietnamese did not have to accept communism to have freedom—may not have been entirely fantastical.

At Geneva, in the midst of the Vietnam crisis, the British Foreign Minister Anthony Eden found himself explaining to his aides the position of the great powers. He used a metaphor from rugby: the Russians were inside left, he told an aide, and the Chinese outside left. He was inside right, and the Americans outside right. This misses the heartbreak of Vietnam, and the tragedy of the war’s trajectory. The Americans, perplexed by themselves, believed they were both outside right and outside left, all at the same time.

Benjamin Wallace-Wells is a contributing writer for the New York Times Magazine and a contributing editor at Rolling Stone.