Jan Karski (born Jan Kozielewski) was a Polish World War II resistance movement fighter and later professor at Georgetown University. In 1942 and 1943 Karski reported to the Polish government in exile and the Western Allies on the situation in German-occupied Poland, especially the destruction of the Warsaw Ghetto, and the secretive German-Nazi extermination camps.
After the war Karski entered the United States and began his studies at Georgetown University, receiving a Ph.D from the institution in 1952. In 1954, Karski became a naturalized citizen of the United States. He taught at Georgetown University for 40 years in the areas of East European affairs, comparative government and international affairs. Among his students was Bill Clinton (Class of 1968). In 1985, he published the academic study The Great Powers and Poland based on research on a Fulbright fellowship in 1974 to his native Poland.
On 2 June 1982, Yad Vashem recognised Jan Karski as Righteous Among the Nations. A tree bearing a memorial plaque in his name was planted that same year at Yad Vashem's Avenue of the Righteous Among the Nations in Jerusalem. In 1994, Karski was made an honorary citizen of Israel in honor of his efforts on behalf of Polish Jews during the Holocaust (Shoah). Karski was nominated for the Nobel Prize and formally recognized by the UN General Assembly shortly before his death.

Source: http://en.wikipedia.org/wiki/Jan_Karski