When Charlie Payne entered the camp he saw a pile of machine-gunned prisoners, sheds full of stripped corpses, stacked on top of each other, waiting to be burnt.

“I don’t know how many, but many high and the whole length of the room,” said Mr Payne, 84. “They sprinkled lime to keep the smell down.” Mr Payne was one of the first Americans to witness the consequences of the Holocaust when his infantry unit stumbled on the sub-camp of Buchenwald in April 1945.

Today, his great-nephew, President Obama, will visit the camp that Uncle Charlie helped to liberate, thus countering Holocaust-deniers such as President Ahmadinejad of Iran who this week called the Holocaust “a big deception” and denounced the US for its support for Israel.

Mr Obama travels this morning with Angela Merkel, the German Chancellor, to the camp where more than 56,000 people were killed by the Nazis, then to a US military hospital in Landstuhl, western Germany, to visit soldiers wounded in Iraq and Afghanistan, before heading for the D-Day ceremonies in France. The message: that war can be justified in defence of humanity but that he is aware of the price paid on the ground.

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The branch of Buchenwald freed by Mr Payne and the 89th Infantry was called Ohrdruf, about 25 miles from the main camp. Buchenwald prisoners were being used there to dig a labyrinth of tunnels and bunkers to house Hitler and the Nazi leadership should Berlin have to be evacuated.

“The prisoners died like flies,” said Volkhard Knigge, the director of the Buchenwald museum, “Everyone was dispensable. You were worked to death.” He calculates that 10,000 died in Ohrdruf between the autumn of 1944 and the spring of 1945. The President will visit the main camp, where he will pay tribute to those killed and to the inmates who protected 904 Jewish children from being slaughtered.

Buchenwald forms a key part of US understanding of the Holocaust — the radio reporter Ed Murrow broadcast from there and Generals George Patton and Dwight Eisenhower hurried to inspect the mass graves — just as the British liberation of Belsen shaped opinion in the UK.

But as Dr Knigge said in an interview with The Times, the history of the camp does not make for an easy narrative. It was set up as a camp for political prisoners and many of its former inmates went on to become leading intellectuals including the Bruno Bettelheim, the child psychologist; Imre Kertesz, the Nobel prize-winning novelist; and Elie Wiesel, the Nobel peace laureate and Nobel Peace prize winner.

Communists played an important part in organising the prisoners at Buchenwald. “Later the East Germans would claim that the Americans delayed the liberation of the camp in order to give the SS the chance to kill off a future Communist elite,” said Dr Knigge, “but this was nonsense, they were just unable to admit that anyone but the Soviet army played a role in the liberation.”

Ohrdruf was liberated on April 4 but the Americans reached the main camp of Buchenwald only several days later. The reason, says Dr Knigge, was the need for the US troops to secure positions and to take care of the thousands of survivors they had found in Ohrdruf.

After the war, and the withdrawal of US troops from eastern Germany, the Soviet secret service, the NKVD, set up a special camp for anti-communists and suspected war criminals on the site of the Nazi camp. “This has encouraged the far Right to claim that Buchenwald was just a place of Stalinist crimes, undeserving of any kind of memorial,” said Dr Knigge.

“This is a place that demonstrates 20th-century barbarity in all its complexity. That will be the message of the Obama visit: Buchenwald is the counter-argument for all forms of totalitarian ideology.”

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