Lyndon Baines Johnson was either the best of presidents or the worst. His War on Poverty and his civil rights achievements were breathtaking in their ambition and lasting impact. Because of his powerful sense of social justice and matchless political skill, we have Medicare today, and Head Start, and a Voting Rights Act that has empowered African-Americans like Barack Obama to succeed in an American political landscape that had once been rigged against them. And yet LBJ's legacy will always be haunted by his failures in Vietnam.

Historian Charles Peters, who actually worked in the Johnson administration, insightfully and accessibly explores both Johnson the man and his political contradictions. Peters asks the central question of LBJ's life: "Why then did Johnson decide to escalate the [Vietnam] war. ... Why would he risk spoiling the incredible record of domestic accomplishment" by pushing the United States into a quagmire in Southeast Asia? Perhaps the ultimate irony is that Johnson knew the war was going badly, yet he was terrified that his political opponents (such as Robert Kennedy and Barry Goldwater) would label him as "soft on communism."

At his best, fighting for equality and social justice, Johnson would forever change the nation with his energy, compassion, and political genius. On civil rights, Johnson understood that pushing lasting legislation through Congress would result in the Democrats conceding the South to the Republicans for decades. The Texan Johnson was correct in his political calculations, but he still did the morally right thing despite the cost.

Peters offers us several examples of LBJ's political genius. For instance, after watching Alabama state police fail to protect Martin Luther King's peaceful marchers from violent segregationist mobs, Johnson invited Alabama Gov. George Wallace to the White House. Peters memorably describes the tense 1965 standoff. The massive Johnson towered over the diminutive Wallace. LBJ, writes Peters, showed Wallace "a picture of a state trooper kicking a black marcher, saying, 'I know you're like me, not approving of brutality.' The president continued in the same vein" by asking Wallace if he wanted to be remembered as someone who hated. After the meeting, writes Peters, LBJ led the cowed governor in front of reporters: "Johnson told them that Wallace had agreed that blacks should be registered and marchers protected." The beaten Wallace offered no protest.

In this outstanding biography's most important revelation, Peters explains why Johnson continued his failed policies in Vietnam. Peters offers us Johnson's darkest fears in the Texan's own words: Opponents would think "that I had let democracy fall into the hands of the Communists. That I was a coward, an unmanly man, a man without a spine. Oh, I could see it coming all right. Every night when I fell asleep." LBJ's compassionate dreams were matched by powerful nightmares.

Chuck Leddy is a member of the National Book Critics Circle and reviews books regularly for the Boston Globe and B&N Review.