After more than seven decades of covering conflicts around the world, Don McCullin will return to Vietnam and his best-known images for his final book.

The photographer, who got his start aged 23 when his image of a gang in Finsbury Park was published in the Observer, has decided to revisit the war and his 12-day stint with US marines during the battle of Hue in 1968.

McCullin’s photographs from the battle, including a shell-shocked American soldier, are among the most iconic images of the conflict and widely credited with helping to turn public opinion in the US against the war.

McCullin said he was still haunted by some of the photographs he took during one of the bloodiest and most notorious battles of the Tet offensive, which he described as “total madness and insanity”.

“They bother me at night when I go to bed,” he said. “They come uninvited back to me and then I start saying: ‘Could I have done better? Could I have done this or done that?’ The actual battle I was in, the final big battle I was in 1968. I saw an awful lot of American soldiers getting killed very close to me.”

One of his editors at the Sunday Times was Harold Evans, who said McCullin’s secret ingredient was empathy for his subject – whether that was criminals in north London or guerrilla fighters in central Africa. He combined a “cold eye informed by the warmth of his empathy”, according to Evans.

Before he was dispatched to Vietnam, McCullin had cut his teeth shooting conflicts in the Congo in the bloody, chaotic lead-up to independence and in Cyprus, where he covered the civil war between Turkish and Greek factions. He was also present as the Berlin Wall was being built and the iron curtain was erected across Europe.

He retired from war photography aged 75, when he visited Aleppo in Syria and was no longer mobile enough to quickly duck out of harm’s way if needed. Since then he has published books about his other passions, including ancient Rome.

So why go back to that war for his final book, which is called Vietnam. Why not revisit his work in Belfast or Biafra? “Because of all the wars that have been raging in the last 20, 30 or 40 years, there was no war like Vietnam,” he said. “Sadly, 58,000 American soldiers died and 300,000 were wounded. It was an extraordinary American misadventure.”

McCullin made 16 trips to Vietnam. His new book includes 100 images and the accoutrements of war, including his helmet with “Times England” emblazoned on the side and his muddied compass.

McCullin, who is now 91, is arguably the most celebrated living British photographer. He has had a retrospective at Tate Britain, touring shows of his work have crisscrossed the globe and he has continued to work from his base in Somerset.

While his photography often brought the horrors of war to wider public attention and in many cases helped to build a moral case for ending conflicts, he is not convinced about the power of his images to effect change.

“I’ve made absolutely no impact whatsoever,” he said. “I’ve come away with these pictures and they were published and people were shocked to see them. But look at the wars that have happened since the Vietnam war. They’ve all been misfortunate too. I’ve been to many wars in Lebanon, and it’s still going on. How could I make a difference, really?”

The book will be released in October via Gost books.