Jan Karski was born Jan Kozielewski to a Roman Catholic family in Łódź, Poland. After graduating from a military academy, he served with a Polish army mounted artillery regiment. In 1935, he received a master's degree in law and diplomatic science at the University of Lviv (Lwow), and then served in various junior diplomatic posts in Germany, Switzerland, and the UK from 1936 to 1938. In 1939, when the Soviet Union invaded Poland in the prelude to World War II, he was among the Polish officers, policemen and leading citizens rounded up and taken prisoner. Two months later he escaped from the POW camp, and returned to Poland, where he joined the armed resistance movement against the Nazis. He repeatedly crossed enemy lines to act as a courier between Occupied Poland and Western Europe. He was smuggled into the Warsaw Ghetto and the Izbica transit camp by the Jewish underground in order to witness them first-hand and report to the outside world. In 1942, Karski was dispatched to the Polish government-in-exile in London and the Allies to describe the destruction of the Jews in Poland and appeal for intervention. In 1943, he personally met with President Franklin D. Roosevelt in the White House. In 1944, he published Story of a Secret State: My Report to the World, which became an instant bestseller, and went on an extensive speaking tour of the USA and Canada. However, his mission to stop the Holocaust failed. After World War II, he emigrated to the USA and earned a PhD from Georgetown University. He taught history at Georgetown for 40 years, specializing in East European affairs, comparative government and international affairs. He also went on many more international speaking tours, this time sponsored by the U.S. State Department, and often testified before Congress on Eastern European matters.
His other books included The Great Powers and Poland: From Versailles to Yalta (1982).