Ten years ago, not long before the election that put Barack Obama in the White House, I went to Washington to interview Seymour Hersh, the reporter who, in 1969, single-handedly uncovered the atrocities that had been committed by an American platoon in My Lai, South Vietnam, 12 months before: a story that hastened the end of the Vietnam war and for which, in 1970, he won a Pulitzer prize. I remember our encounter vividly: the chaos of his office, with its filthy walls and toppling piles of notebooks; the unstoppable flow of his conversation; the wolfish greed with which he scoffed his eggs at breakfast. Above all, what has stayed with me was his almost total lack of interest in anything other than his reporting (by his own estimation, pretty brilliant); his contacts (so numerous that they rival the crowd at the Super Bowl); his editors (occasionally fantastic, but more often annoying and dumb). If Hersh had a hinterland, he was keeping it well hidden.
Thanks to this, I was well prepared for Reporter, a memoir he only embarked upon because the book he was contracted to write for his publishers – a coruscating volume about Dick Cheney and all who sail in him – had hit the buffers. I guessed – correctly, as it turned out – that Reporter would be unrelenting, and focused entirely on his work; that it would come with no false modesty (or much modesty at all). All the same, even I was taken aback by the extent of his completism. Did he kidnap the book’s editor, tie him or her up until it was at the printers? It’s right that the big stories he has reported – My Lai, the domestic and foreign policy crimes of the Nixon era, the abuse of Iraqi prisoners by US soldiers at Abu Ghraib – are recounted in chapters that run to many pages. But it seems a bit much when, determined not to leave anything out, he resorts literally to running lists of the other, smaller scandals on which he worked in between. Detail swamps his narrative, like creeper clambering over an ancient Mayan ruin, and for the reader, hacking through it is completely exhausting.

It is not, however, dispiriting – or not all the time. Here is journalistic tenacity of a kind that, social media being what it is, is close to nonexistent today. The story of how Hersh, then a broke freelance, stumbled on the appalling events at My Lai is familiar by now: when a military lawyer told him that a soldier at Fort Benning in Georgia was facing a court martial for killing at least 109 Vietnamese civilians, Hersh simply rocked up at the base and went door to door until he found 26-year-old Lt William L Calley Jr (he later followed this up with an even more amazing interview, this time with Paul Meadlo, a farm kid from Indiana who had shot many of the civilians before losing a leg himself). Reading about it here, though, you’re reminded all over again of just how hard it was to get such a scoop published. The first report was rejected out of hand by many media organisations, among them the New York Times, and carefully rewritten – Hersh sold it through a tiny agency – by others seemingly made nervous and resentful by it.
All the Vietnam movies in the world, moreover, cannot lessen the impact of My Lai; 50 years on, its horrors will not be varnished. Hersh’s excitement at the trail, and what it might mean both for his career and those of the warmongers, is tempered here by details that stay with you long after you put the book down: the sight of Calley, on the night the two men first met, vomiting blood, the result of a serious ulcer; an account, given to Hersh by a GI who’d shot himself in the foot in order to get out of My Lai, of a soldier who used his bayonet to toss a little boy in the air as if he were a “papier-mache piñata”. Hersh might be a monomaniac, but he deserves all the respect in the world for the work he did then. Determination, even obduracy sometimes, directed unyieldingly at politicians and other high-ups, is never not a good thing – something that, as he notes, his more craven colleagues in the US press would do well to remember. Judge a hack by his enemies (Kissinger was one of his), not his friends. Journalists must arm themselves with facts, not settle into tired punditry. “I think, I think…” For Hersh, these are the deadliest words in the language.
Hersh’s parents were Jewish immigrants to Chicago, and when his father died in 1954, a month after his son had graduated from high school, it seemed likely that he might spend the rest of his days running the family cleaning business. He was saved by a college teacher and after graduating from university, joined a local news bureau as a copy boy. Apart from a brief period when he worked as press secretary to the Democratic senator Eugene McCarthy – surprise, surprise: they fell out – he has never not been a journalist. After winning his Pulitzer he joined first the New Yorker, then the New York Times; he would later return to the former under Tina Brown. Did he make editors like her nervous? Just a little. Brown once told him that the US secretary of state Colin Powell had informed her, over dinner, that Hersh was a liar. He heard the edge in her voice, but reminded her that such a comment was “a badge of honour” for someone who’d never been invited to a White House junket and never wanted to be. (The closeness of editors to politicians, whether of left or right, appals him.)
He’s not always sure-footed. In the 90s, when he was working on a book about Kennedy, he was taken in by some documents that purported to show that the president was being blackmailed by Marilyn Monroe; the fact that he discovered this in time to remove all mention of them didn’t stop his critics from piling in. But you have to say that his instincts are mostly great. Hersh’s laws of journalism, which will, alas, never be taught on any media studies course, are something to live by. Sometimes, he writes, a story just “smells” right: this was certainly his feeling when an Iraqi general first told him that under the US occupation, women prisoners were being assaulted by guards to the point where, regarding themselves as dishonoured, they would write to their fathers and beg them to come and kill them. But still, it is important to hold your scepticism, if not your cynicism, close. Or as he puts it: “If your mother says she loves you, check it out.”
• Reporter: A Memoir by Seymour M Hersh is published by Allen Lane (£20). To order a copy for £17 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99