Shaped By War: Photographs by Don McCullin, Imperial War Museum North, Manchester

Some years ago, Don McCullin gave a lecture to a room of aspiring photographers. "How many of you think of yourselves as photographers?" he asked. All raised their hands. "How many of you have got your cameras with you right now?" he said next. None raised their hand. "Well you're not photographers then."

Wherever McCullin went, he took his camera, and where he was going was usually a war zone. The results can be seen at the Imperial War Museum North's major retrospective of McCullin's photographs from conflicts around the world. They suggest a man who seemed always to be in the right place at the right time – his first cover story on Vietnam appeared the week the first US Marines arrived in the country – but this was no accident: it was hard work, guts and instinct.

His breakthrough was typical: a dynamic picture of seven schoolfriends, north London toughs called the Guv'nors of Seven Sisters Road, dressed in Sunday finery and draped with adolescent brashness around the frame of a ruined building.

When one was hanged for killing a policeman, the self-taught McCullin sold the picture to The Observer. "The jumping-off point for my whole life was based on a terrible act of violence and its consequences," he writes in the book that accompanies the exhibition.

In 1961, while on a belated honeymoon, McCullin spied Peter Leibling's picture of an East German soldier skipping the wire to freedom, and promptly announced he was off to Berlin. A lifetime as a "war junkie" began, mostly for The Sunday Times Magazine.

The strict chronology of the exhibition – which also features letters, objects and cameras – shows how McCullin's interests developed. His early photographs focus on combatants, most famously in Vietnam, and even stray into artistic iconography, something he claims to dislike. One image shows a Turkish Cypriot crouched and clutching a Sten gun, a Mediterranean Dillinger casting a striking shadow against a whitewashed wall. He admits it looks like "a movie still".

These early images – of handsome US Marines and teenage gangsters – are the only ones that hint at romance. Contrast comes later, when, in 1982, we see gloating Christian Phalangists in Beirut. Set against derelict buildings and posing cockily, they look much like the Guv'nors, but with all the grubby glamour scrubbed away.

He later preferred to photograph victims of war, although these do feature in his early work. One image, of a grieving Cypriot widow, earned him his first award, something that still makes him uncomfortable, even angry. For he is not an impassive man, and claims never to have stopped trying to make sense of the guilt he feels both as a survivor and as an active observer of hideous events.

"I'm ashamed of it, of all the things I've seen, of all the blood, of all the burnt children," he writes. "I'm disgusted with the whole business, but I did it for 30 years."

There is only one photograph on display that makes you wonder how any sane man could have taken it. It's of a weeping Bangladeshi child, flat in the mud, staring at the camera with desperate, unseeing eyes. The image that haunts McCullin is of another child, a Biafran albino made grotesque by starvation. "When I'm in the darkroom and that image is coming up, it's as if he's saying "Hello! Hello, I'm back!", he writes. "I think it's one of the worst pictures I've ever taken."

McCullin visited Vietnam 15 times and almost died in both Cambodia and El Salvador, but two of his most memorable photographs come from closer to home. In Northern Ireland, he captured a housewife reeling back in terror into her suburban doorway as a phalanx of British soldiers storm a Derry street. Her eyes are fixed on them, theirs on a more distant target. Another shows a gaggle of ragged Bogside teens leaping a barricade, shrouded in CS gas which renders them like Dickensian urchins emerging from a London fog.

Although he never really retired – he covered the first Gulf War for The Independent in 1991 – the glory years ended bitterly. McCullin was barred from the Falklands – key correspondence is displayed – and then sacked from The Sunday Times. Now he shoots Roman ruins and landscapes, but still in black and white, and one can see why when compared with the handful of colour images featured. These are still fine photographs, but with none of the depth that distinguishes his greatest work. One, of a shell-shocked American soldier, is covered in McCullin's contemporary notes on how it should be reproduced. "This print should be much darker," it says. For McCullin, one wonders if it could ever be dark enough.

The exhibition runs at IWM London from 7th October 2011 to 15th April 2012