Peter had no idea that when he walked through the sturdy oak door, his world would change forever.

It was spring, 1994; he was a few months short of his fortieth birthday, and he was in Cologne — called Koeln by the natives — visiting his chess friend Joerg, whom he had met a couple of years previously at the annual Hastings chess congress. He was staying at Joerg’s house, and they were both playing in Koeln’s local chess tournament. Today was the rest day, and Joerg was showing him around his home city.

They had enjoyed an excellent lunch, complete with three bottles each of the famous local Koelsch beer that could only be brewed within the city. They were both feeling relaxed. As they had exited the restaurant, Joerg had spotted, just a few yards up the road, the Koelnische Stadtmuseum. He had suggested that, as Peter was keen to find out more about the city, they should take a look.

The city of Koeln dates from Roman times in the first century AD. The museum has a lot of history to display, so the building is large. They both spent a happy couple of hours wandering around the exhibits. Joerg was familiar with the museum, so he acted as a guide.

Late in the afternoon they stopped beside a large oak door that was labelled as leading to an exhibition of Koeln during the years of the Second World War.

“You may find that section unpleasant,” said Joerg. “What the Nazis did during those years was very bad, and it ended up causing the ruin of our beautiful city.”

Peter said he was keen to see the entire history, both good and bad, and he must nonetheless have a look. Rather grudgingly, Joerg opened the door. The room they entered was not very large. The opposite wall featured only a huge photograph, and it gripped Peter’s attention right away. He had never seen such a photo before. It wasn’t, as he had imagined it might be, a picture of starved, diseased and almost naked people in a concentration camp. He had seen such pictures before, and they had made him shudder. He knew what atrocities the Nazis had committed in those years before he or Joerg was born.

Instead, what he was looking at was an aerial photograph of an almost completely flattened city. The only standing building, right in the middle of the picture, was an enormous church. Peter recognised it immediately as the Koelner Dom, the cathedral of Koeln, from one tower of which they had looked out over the city that very morning. It had held the title of the tallest building in the world between 1880 and 1890.

Peter looked to the side of the photograph to read its description. It was in German, so Joerg quietly translated for him, explaining that throughout the war, Cologne had been bombed 262 times. Nearly two-thirds of the city had been destroyed, and around 20,000 civilians killed. One raid alone, in May 1942, involved more than a thousand RAF bombers and left huge areas of the city in flames.

Joerg added a detail not written: “My grandfather told me he thought the only reason the Dom was still standing is that the Royal Air Force needed it as a navigation point. If they had flattened that too, the soul of our city would have died.”

Peter looked over at Joerg, his eyes brimming with tears. “I am so sorry,” he said. “I had no idea my country had done that to your country.”

“We started it.” stated Joerg.

“Of what possible relevance is that?” Peter asked.

“Very relevant, I would have thought.”

“No civilians, under any circumstances, should be treated like that. I cannot believe that my country did that.”

Like many Germans of his generation, Joerg had been well taught to be ashamed of his country’s behaviour in the past. They looked at the British raid as deserved punishment.

Wiping the tears from his eyes, Peter asked another question. “You people have rebuilt your city?”

“It took many years. We rebuilt it as it had been before, even the historic buildings and churches. You may have noticed vacant lots in various places in the city?”

“I did. I was going to ask about them.”

“They are our war memorials. We see them every day as we go about our business.”

“You are wonderful people, Joerg. Don’t let anybody tell you any different. What you have done here must be one of the greatest acts of reconstruction anybody has ever attempted.”

Peter never forgot that photograph, or the unguessed realities that it portrayed. He played in the rest of the tournament, but his heart wasn’t in it. He lost all his remaining games. Not even chess, the beautiful game he was obsessed with, could lift his depression.

Peter had always been interested in history, but until that day it had been little more than a hobby. He remained on the other side of that door for the rest of his life.