When I first heard the eulogy in 2006 for my great-uncle Hanns Alexander, I was amazed. Apparently, he had been served as a Nazi hunter in the British Army at the end of the Second World War. How was this possible? After all, he had grown up as a Jew in Berlin.

Like so many other European Jews in the 1930s, the nineteen-year-old Hanns and his family had fled Nazi persecution and had arrived in Britain in 1936. His first task was to learn the language and find a job.

But when war was declared in September 1939 he immediately signed-up for military service. He felt that it was his duty to fight for the adopted country which had so generously provided refuge to his family. It would also be a chance to wreak revenge on the Third Reich who had driven them out of Germany.

Yet, Hanns’ offer to fight for his adopted country was not immediately accepted. The British government was uncertain about how to deal with applications from newly arrived German and Austrian refugees. Officially it welcomed all who volunteered and were fit for service, but it was wary of taking in men who it feared might pursue espionage or sabotage.

It wasn’t until December 1939, three months later, that the Hanns received word regarding his enlistment: Hanns was to be part of the Auxiliary Military Pioneer Corps and was ordered to report at once. He was given the army number 264280.

The Auxiliary Military Pioneer Corps had been created on 17 October 1939 to make use of men who were refugees from Germany and elsewhere who wanted to fight Hitler. For these men the stakes were high. If caught by the Reich, they would be viewed as traitors and shot. Yet, of the more than 70,000 German and Austrian refugees who landed in Britain between 1933 and 1939, approximately one in seven enlisted with the Pioneers.

At the Kitchener training camp on the south coast of England, Hanns joined up with other refugees who had been assigned to the Pioneers. Housed in dilapidated barracks that let in snow through the holes in the roofs, the Pioneers jokingly called the place ‘Anglo-Sachsenhausen’, after the Berlin camp in which some of them had recently been held.

Few of the volunteers spoke the local language, yet they insisted that they swear allegiance to the King in English: “I certify that I understand the risks . . . to which I and my relatives may be exposed by my employment in the British Army outside the United Kingdom. Notwithstanding this, I certify that I am willing to be employed in any theatre of war.”

But Hanns and his comrade’s hopes that they would be soon fighting the Nazis were soon disappointed. For they learned that the British Army trusted them enough to don the official uniforms and swear allegiance, but not enough to equip them with guns.

The Pioneers would spend the next few years digging trenches, loading trainings and acting as orderlies in field hospitals. Even when it came for the big push into Europe, in June 1944, spearheaded during the Normandy Landing, these Pioneers served only in non-combat roles.

Shortly after being deposited on Gold Beach near the small town of Arromanches, Hanns and his company were asked to supervise a small group of German officers who had been captured the day before. They held the POWs in an open field without barbed wire. The only boundaries to this makeshift camp were white anti-mine ribbons that they had strung between some trees. These German Jews, who had been forced out of their homes and were now wearing British uniform, were tasked with controlling their oppressors. It was a strange, awkward situation.

The British attitude towards the Pioneers changed with the approach of the war’s end. It was as if they suddenly realised the incredible resource they had at their disposal, for these refugees knew the language, the culture and the lay of the land better than anyone else.

In May 1945, Hanns was assigned to the 1 War Crimes Investigation Team and sent to a camp in the North of Germany to help with interrogating some guards who had been captured. The name of the camp was Belsen.

It was early evening on 12 May 1945 when they arrived at the barbed-wire gates of Belsen. Inside the camp, corpses lay piled on top of each other. The living prisoners were so thin that their ribs poked through their skin. Mothers clutched dead children; shaven-headed survivors in black-and-white-striped uniforms stared vacantly by decrepit wooden barracks; painted signs warning of typhus epidemics were everywhere. There was no water, no food, inadequate medical supplies and little shelter.

Hanns’ first impressions of Belsen were visceral: “Before it came to interpreting it was a question of cleaning the camp out. Everybody did whatever they could. There were dead bodies walking about, dead bodies lying about, people who thought they were alive and they weren’t. It was a terrible sight.”

Hanns’ first task was to help bury the corpses strewn across Belsen’s grounds. With the help of other soldiers – one holding the legs, the other grasping the arms – Hanns carried hundreds of bodies to a mass grave.

All the British soldiers were deeply disturbed by what they had found in Belsen. But Hanns’ reaction was different. The atrocity at Belsen had happened in the country of his birth; its victims were mostly Jews, his people. He could understand the German-speaking prisoners, people with whom he shared a context and background. Their story could so easily have been his. For Hanns, this was his home, and there would be no respite. It was as if Belsen had tripped a switch in him. No longer was he a carefree, selfish young man. He was gripped by a barely controllable rage. And he sensed a purpose.

Hanns approached his commander and suggested that he search for senior Nazis who were beyond the camp fences, hiding somewhere in North Germany. After some negotiation, he was soon spending his time tracking down the perpetrators of the Final Solution.

To my surprise, as I continued my research, I learned that my great-uncle had not only been a Nazi hunter, he had been one of the first, if not the first in the British Army.

During his time he interrogated the Kommandant of Belsen, the doctors who oversaw the selections in Auschwitz-Birkenau, he tracked down the leading Nazi in Luxembourg, the Gauleiter of Luxembourg and, after an extraordinary chase through Northern Germany, the Kommandant of Auschwitz.

It seemed amazing, but it turned out to be true: it was possible that you could be born a Jew in Germany, and serve as a Nazi Hunter in the British Army at the end of the Second World War.

Thomas Harding is author of “Hanns and Rudolf: The German Jew and the Kommandant of Auschwitz” (Windmill; paperback available May 2014). He will be speaking at CVHF 2014 on this subject.

You can follow Thomas on twitter @thomasharding