The letter was addressed to “Captain W Merkel, Featherstone Park Camp, Northumberland” and dated October 14, 1946.

Featherstone Park was a prisoner of war (PoW) camp. The letter’s author, Kurt Schilling, was a former Nazi officer who had just been released from the camp and returned to Germany.

“I cannot but thank you for all the kindness and humane understanding you showed, not only to myself, but also to the other PoWs in ‘C’ compound,” he wrote to Captain Walter Merkel.

“I myself am grateful to you and those officers like you who made my unpleasant duty much easier by their excellent understanding of the mentality of prisoners of war.

“Most of those British officers under the command of Colonel Vickers have done more for understanding between our two nations, by the way of treatment in the camp, than statesmanship can ever hope to achieve.”

Featherstone was one of a number of PoW camps operated in Britain to rehabilitate German soldiers, including many devoted Nazis.

What made it different from any other camp was that its key officers — Colonel Vickers, Cpt Merkel and a Captain Herbert Sulzbach — were all Jewish.

Robert Bieber, a visiting research fellow at the department of war studies at Kings College, London, came across the story via his wife’s work at Richmond Synagogue.

He explained: “My wife runs a day centre at the shul for elderly people, and one of the ladies who’s a member came to me and said: ‘I’ve got some very interesting papers. My first husband was in the army, and I thought you’d like to see them’.”

The papers contained thank-you letters to Cpt Merkel, as well as other items that the German PoWs had made to show their gratitude towards him, including a specially designed Christmas card, and a book full of expertly drawn cartoons depicting life at Featherstone and Cpt Merkel’s role at the camp.

The portfolio of letters and drawings was acquired by the Imperial War Museum earlier this month, with the collection described as “unique and visually arresting”.

“Herbert Sulzbach wrote a book. His role was reconciliation,” Mr Bieber said.

Cpt Sulzbach, a British officer in World War Two, had been a German Officer in World War One, winning the Iron Cross first and second class for bravery.

He would go on to be awarded with both an OBE and the Croix de Guerre, possibly giving him a unique collection of medals. He also received letters from grateful prisoners, including members of the SS.

One wrote of Cpt Sulzbach’s rehabilitation efforts: “We were all the more astonished that you did not exclude us members of the SS, who should inevitably have been your enemies to the death.”

Another, an SS Standartenfuhrer, wrote to the captain saying simply: “You have cured me of certain preconceptions.”

Mr Bieber said: “Cpt Merkel’s role has been far less recognised. But it’s recognised here in terms of the artefacts. The Imperial War Museum want to make these particular items a centre-piece of their new World War Two exhibition.”

Mr Bieber described the relationship between Cpt Merkel and the Nazi officers as a “remarkable rapport”.

“They never suppressed the fact that they were Jewish. Cpt Merkel, in conjunction with Cpt Sulzbach and Col Vickers — all three of them were Jewish — built a remarkable system.”

Despite his surname, Cpt Merkel was born to a British family, in Newcastle.

“He was one of six, an identical twin,” his daughter Karen said. “When the depression came, the whole family moved from Tyneside to London.

“My father left school when he was 14. He and his twin joined the Jewish Lads Brigade in London and also the Territorial Army. They were keen to serve. He joined the Durham Light Infantry, but was ill and in hospital having an operation when they went off to Burma and never came back, so it was extraordinary that he survived, like a flip of the coin.

“He was then posted back up to Northumberland to serve at the prisoner of war camp for senior Nazis.”

Cpt Merkel met and married his wife while an officer at the camp.

“It was extraordinary that he found himself one of three senior officers, all of whom were Jewish,” his daughter said.

“I don’t know what the odds are of that, but it does seem quite extraordinary that they were there, looking after senior Nazi officers.

“They got on. Obviously it was their job to encourage the officers to think differently. They were there to encourage new ways of approaching life and thinking.

“We have some really stunning items in the family which the prisoners made for my father. We have a clock, candlestick holders, the most beautiful big bookcase, an ashtray, jewellery boxes, and the most amazing box for my mother when they got engaged.

“There was a huge affection.”

The three British officers at Featherstone were not the only ones who worked in camps designed to aid in the de-Nazification of German soldiers.

An article written by Dr Anthony Grenville for the journal of the Association of Jewish Refugees, in memory of Cpt Sulzbach, noted an incident recounted by a former German prisoner at Camp 180, near Cambridge.

An SS officer at the camp shouted “Jew lout” at Charles Stambrook, a British Jewish officer, as the prisoners were being counted.

“Let us reflect for a moment what an SS officer would have done, if a prisoner of war had shouted ‘SS lout’ at him,” the prisoner said.

“This is what the British officer did. He turned round cooly and said calmly to the man who had shouted: ‘The Jew part is correct, the lout part isn’t’. And carried on.”

Mr Bieber said he “couldn’t imagine what came over the British authorities, to put three British Jewish officers in charge of a Nazi prisoner of war camp.

“I had a sense of almost disbelief to read about it in the first place, but then a sense of ‘isn’t this what Jewish people stand for? Reaching out the hands of friendship?’. This is exactly what Jewish people do. So I felt a huge sense of pride and belief, justifying my confidence in what we Jews are.”