During the war, Karski worked for the Polish Underground State and served as a liaison between the Polish resistance movement and the Polish government-in-exile in London. He was twice sent on dangerous missions to Nazi-occupied Poland, where he gathered information about the Nazi genocide of the Jews.
On his first mission, in 1940, Karski witnessed the horrors of the Warsaw Ghetto, where he saw Jews living in squalid conditions, starving to death, and being subjected to torture and murder by the Nazis. He also met with members of the Jewish underground, who provided him with information about the Nazi plans to exterminate the Jews.
On his second mission, in 1942, Karski visited the Belzec extermination camp, where he witnessed mass killings of Jews. He was smuggled into the camp by a member of the Polish resistance movement, who disguised Karski as a Ukrainian guard. Karski was able to see the gas chambers, the mass graves, and the piles of bodies. He also spoke with survivors of the camp, who told him about the atrocities they had witnessed.
After returning from his missions, Karski wrote a report about his experiences, which he presented to the Polish government-in-exile and to the Western Allies. He also met with leaders of the Jewish community, including Rabbi Stephen Wise, and shared his information with them. Karski's reports were instrumental in raising awareness of the Holocaust and mobilizing support for the rescue of Jews from Nazi Germany.
In recognition of his courage and dedication, Karski was honored by Yad Vashem, the World Holocaust Remembrance Center, as one of the Righteous Among the Nations. He was also awarded the Presidential Medal of Freedom, the United States' highest civilian honor.
Jan Karski's story is a powerful reminder of the horrors of the Holocaust and the courage of those who risked their lives to help save others. His legacy is one of hope and inspiration, and his story continues to be told around the world.