In the summer of 1942, Jan Karski, a 28-year-old courier for the Polish underground, was instructed to undertake an extremely dangerous mission: a trip across Nazi-occupied Europe to London, where, with the help of the Polish government-in-exile there, he was to brief top Allied officials about what was happening in his native land. He was then to continue his journey to Washington, where once again he was supposed to meet with the highest officials.

Before his departure, Karski gathered messages from the leaders of various underground organizations that he would convey to the West, including from two leaders of the Jewish resistance who had managed to slip out of the Warsaw ghetto to meet him in a half-ruined house on the outskirts of the city.

Karski was well-suited for his mission. Growing up in the manufacturing city of Lodz, he was a devout Catholic who adhered to his mother’s teachings about respecting those of other faiths, particularly the city’s large Jewish population. Before the war, he had spent time in Germany, Switzerland, and England, all part of his preparation for an anticipated diplomatic career. He also did his military service and was called up again on the eve of the war. When the occupation started, he quickly joined the rapidly growing underground.

As Karski recalled both in his powerful book The Story of A Secret State, published in the United States in 1944, and in an interview with me in 1998 when he could reveal some additional details, the Jewish leaders he met before his departure to London came straight to the point. “Hitler has decided to murder all the Jews in Europe,” one of them told him. The other leader started to cry. “We cannot hope that the Poles will help us. Poles can save individuals but they cannot stop the destruction of the Jews. Approach as many people as possible—the English, the Americans, whoever. Tell them that we are dying.”

The other message: The Allies need to scatter leaflets across Germany holding the entire population responsible for this mass murder, telling them they would face wholesale reprisals. They should also publicly execute Germans, “any they can get hold of” anywhere in the world.

Karski replied that such retribution was impossible to imagine, and the demand would horrify everyone. One of the leaders conceded: “We do not dream of it being fulfilled, but nevertheless we demand it,” since this would demonstrate “how helpless we are, how desperate our plight is, how little we stand to gain from an Allied victory as things are now.”

Finally, they told Karski that he should convey their demand that Jewish leaders in the West should go to government offices and start hunger strikes there, not relenting “until they have obtained guarantees that a way has been decided upon to save the Jews.” They should refuse all food and water, dying “a slow death while the world is looking on… This may shake the conscience of the world.”

Karski understood that these Jewish leaders were all too aware of “the complete hopelessness” of their situation, which is why they had cast all practical considerations aside. “For them, for the suffering Polish Jews, this was the end of the world.”

Since no one on the outside could possibly imagine the targeting of an entire people for systematic destruction, the Jewish leaders offered to show Karski as much as possible of what was happening to their people, hoping that his first-hand testimony would lend credence to their appeals. Karski never hesitated, despite the obvious risks this would entail before he even started his journey.

Reaching the Warsaw ghetto by a secret passage through a building adjoining the ghetto wall, Karski saw “a new world utterly unlike anything I could have ever imagined.” There were the naked dead bodies of old men lying in the streets and “a woman walking with her baby at her breast which was no breast,” he recalled. From the window of an apartment, he witnessed a member of the Hitler Youth “looking for a target with the casual, gay absorption of a boy at a carnival.” The street suddenly looked deserted but the boy spotted something beyond Karski’s vision. He raised his gun and fired… “then the terrible cry of a man in agony” as the boy shouted with joy.

“Go back, run away,” a woman in the apartment instructed Karski. “Don’t torture yourself any more.” But Karski returned to the ghetto two days later for a second visit “to memorize more vividly my visual impressions.” He was too shaken during his first visit to feel that he could rely on what he saw then.

Even more dangerous was Karski’s visit, dressed in the uniform of a bribed Ukrainian militiaman, to the Izbica transit camp for Jews destined for the gas chambers in Belzec. The Jewish underground had arranged for someone with a pass for the camp to escort him in, and the guard on duty asked no questions. Karski recounted what happened next:

“The Germans were shooting into the air and pushing people onto a train. There was shouting, chaos, confusion. The cattle cars were filling up with Jews. A soldier tore a child from a mother’s arms and threw the child, just like a sack, over the heads of the people into the wagon. It was horrible. I must have had a sudden nervous breakdown, and I don’t know what I did then. I only know that my guide was shouting ‘Follow me!’ and he was angry. He got me out.”

For his journey to London that started on a train to Berlin, the heart of the enemy, Karski was equipped with the papers of a French laborer allowed to return to his homeland for a vacation. He was also carrying a roll of microfilm with numerous reports from the underground about both the military situation and the mass murders of Jews, appealing for Western help. Karski, who possessed a photographic memory, had plenty of other information to convey as well.

Although Karski spoke French, his accent could give him away. To prevent that, he had a dentist provide him with an injection that made his jaw swell. Karski had already lost several teeth when he was captured and tortured by the Gestapo while returning from a previous mission. Fearing he could not withstand the ordeal much longer, he had tried to commit suicide by slashing his wrists. That action probably saved his life because he ended up in a Polish hospital. The doctors and nurses tipped off the underground that mounted a daring rescue, bribing one of his guards so that they could get him to safety. All of which made it easier for Karski to travel with a handkerchief on his mouth, saying next to nothing as he convincingly feigned extreme pain. He also carried a cyanide pill since he was determined not to be captured alive again.

After taking a long, circuitous route, Karski arrived in London in November 1942. As planned, the Polish government-in-exile arranged meetings with Foreign Secretary Anthony Eden and other senior officials. They clearly were impressed by the courage and determination of the young courier, but that did not mean they were receptive to his message. When Karski met with Lord Selbourne, who was in charge of covert operations to help anti-Nazi movements, the official told him he wanted to tell him a story.

“During the first world war, rumors spread all over Europe that German soldiers in Belgium liked to catch Belgian babies and crush their skulls against the wall,” Lord Selbourne said. “We knew the rumors weren’t true, but we didn’t do anything to stop them. They were good for the morale of our people. You are doing a great service. Try to reach newspaper editors, public opinion.”

For Karski the message was clear—and devastating in its implications. “He didn’t believe me. Those people didn’t believe me. They thought I was exaggerating out of hatred of the Germans. That this was just propaganda.”

Arriving in Washington in June 1943, he encountered similar attitudes. Accompanied by Polish Ambassador Jan Ciechanowski, he met with Supreme Court Justice Felix Frankfurter. His host immediately asked him if he knew that he was Jewish and then inquired about the fate of Polish Jews. Karski responded in detail, describing his trips into the Warsaw ghetto and what he saw in the transit camp.

Frankfurter asked him a few questions about technical matters such as the height of the wall separating the ghetto from the rest of the city. After Karski finished, Frankfurter paced back and forth before sitting down again. “Mr. Karski, a man like me talking to a man like you must be totally frank. So I must say I’m unable to believe what you told me.”

A startled Ciechanowski intervened. “Felix, you don’t mean it,” he said. “You cannot tell him he’s lying. The authority of my government is behind him.”

Frankfurter replied: “I didn’t say this young man is lying. I said I am unable to believe what he told me.”

On June 28, Karski was invited to the White House to brief President Roosevelt. In describing the Polish situation, the courier emphasized a key point. “Mr. President, a distinction has to be made. The Germans persecute my people; they deny us education, send us to concentration camps, they want to make us a nation of slaves. With the Jews, it is different. They want to exterminate them.”

Roosevelt offered no comment. “He asked questions, questions, but not a single question about the Jews,” Karski recalled. The president’s final message: “You will tell the leaders that we will win this war! You will tell them that the guilty ones will be punished. Justice and freedom shall prevail. You will tell your nation that they have a friend in this house.”

After making so many visits to top officials in both capitals, Karski’s cover was blown and he remained in the United States. He wrote his Story of A Secret State, which was a huge critical and commercial success—but then this extraordinary account was largely forgotten. He became a highly respected professor of East European and international affairs at Georgetown University, although for many years he did not mention his personal story to his students.

But his story was gradually rediscovered and, as he began discussing it again, he always downplayed the danger of his actions. Speaking of the underground resistance, he wrote: “For the most part, our work was probably less thrilling, less of an adventure, than the work of a carpenter, and wholly devoid of sensational exploits.” That was only the false note in his descriptions of his experiences, born of an instinctive discomfort of portrayals of him as the hero he truly was.

When I visited Karski in his apartment in Chevy Chase, Maryland, two years before his death in 2000, he was surrounded by the many awards he had collected from Israel, the United States and Poland. But he was not boastful in any way. In fact, his entire demeanor suggested that he felt that in one critical respect his mission had ended in failure. He had been clearly hurt by the refusal of British and American leaders to believe him all those years ago—and that fact still stung at the end of his life. Despite all the tributes that followed, he never claimed success.