Everyone has a favourite president, but I’m suspicious of anyone who says it’s Lyndon Baines Johnson. Yes, he was the politician’s politician – one of the greatest legislators in US history – but he was also a monster. Kyle Longley recounts an episode when journalists were badgering Johnson to explain America’s war in Vietnam. LBJ unzipped his fly, pulled out his sizeable member and said, ‘This is why!’ They say the personal is political, but that’s taking things too far.

Longley’s new book examines a year in Johnson’s life – his final one as president – in microscopic detail. At first, you wonder why he bothers. The opening chapter deals with the writing of the State of the Union Address for 1968, an overlong speech never quoted today. The toing and froing of Johnson and his advisers makes for dull reading. But if you can stay the course as far as the third chapter, you start to get the point. Johnson’s final year in office was a fruitless struggle to get anything done at all: a cycle of trial and disaster. The State of the Union Address, in which Johnson tried to rededicate the nation to social reform, was delivered on 17 January and went down fairly well. On 30 January, the Viet Cong launched the Tet Offensive. For the first time, Americans really contemplated defeat in Vietnam. Johnson’s plans fell apart.

On and on it went. Realising that running for re-election in 1968 would likely kill him, and that he might not win anyway, Johnson went on television to announce, ‘I shall not seek and I will not accept the nomination of my party for another year as your president.’ The wording mattered: Longley says that LBJ originally preferred ‘would not accept’ to ‘will not accept’ because he thought the latter was presumptuous, but was persuaded otherwise. The speech transformed Johnson’s image: the public was elated by this rare act of political self-sacrifice. Now that he was liberated from campaigning, maybe Johnson would actually bring peace to Vietnam.

Well, that was on 31 March. On 4 April, Martin Luther King was assassinated. Riots broke out. One step forward, two steps back.

LBJ regained a little magic in the spring and used King’s death to pass significant civil rights legislation, including the Fair Housing Act, which made it illegal to refuse to let or sell to someone on the basis of their race. The Act defied some basic tenets of the Republican Party, such as free choice, and overcoming white prejudice was a genuine achievement. Nevertheless, Johnson was pushing Middle America further than it was willing to go. On 5 June, Robert Kennedy was shot dead in Los Angeles. Johnson tried to squeeze tough gun-control measures through Congress but fell short; the National Rifle Association said no. In August, as the Democrats tore each other apart, he thought they might ring him from their convention in Chicago and beg him to be their nominee for president. Longley is at his most devastating when describing Johnson hanging by the phone like a lovesick teenager. The call never came. On the night of 28 August, the police assaulted anti-war protesters and made Chicago look like Prague, which had been invaded by Soviet troops the previous week. The president spent the rest of the year chasing a peace that never came. And on 5 November, the voters rejected his hand-picked successor, Hubert H Humphrey, in favour of Richard Nixon. By the end of Longley’s book, January’s State of the Union Address, with its hopes and ambitions, has become a cruel joke.

There are plenty of reasons why 1968 became Johnson’s annus horribilis. South and North Vietnam were resistant to negotiating with each other; Nixon’s crew secretly urged the South Vietnamese to wait until he was in office in order to get a better deal. Who could have predicted the murders of King and Kennedy? The rise of popular conservatism, moreover, meant that the public was more interested in fighting crime than eliminating poverty.

But, to repeat, Johnson was a monster – one of those interesting, three-dimensional monsters that occasionally does good, for sure, but very definitely the author of his own misfortunes.

He humiliated Humphrey, for example, a loyal vice president who backed the Vietnam War despite his own misgivings. To stand any chance of winning the general election, Humphrey needed to reconcile the doves and hawks of the Democratic Party, and he favoured an election platform that would appeal to both. Johnson nixed it. He was too obsessed with defending his war to compromise. And it was his war. As Longley writes, the president often talked about Vietnam as if it were something he’d inherited or had thrust upon him and as if all he’d done from day one was try to end it. Hogwash. When Johnson became president, the USA’s involvement in Vietnam was limited. He escalated it. He was in effect an imperial president, yet he adopted the persona of a powerless civil servant. Yes, his generals assured him of progress and pushed for more men, but there were other voices who told him it wasn’t working, men to whom he should have listened, and the Tet Offensive splashed the writing right across the wall.

Without Vietnam, LBJ’s reputation might well be titanic: he secured the Civil Rights Act and Voting Rights Act, launched a war on poverty and signed more than three hundred conservation measures into law. But Longley’s perceptive and unflinching study concludes that Vietnam is inseparable from Johnson. The fact is that he committed America to a gruesome war in defence of its own metaphorical manhood and determined the manner in which it was fought, which was neither total war nor a surgical intervention.

There is a story that tells of a young man trying to guide Johnson to the presidential chopper. ‘There’s your helicopter, sir,’ he said. The president replied, ‘They’re all my helicopters, son.’ It was an answer typical of an outsized man who filled an outsized office, both of which had accrued far more power than the American people should ever have tolerated.