In 1939, Karski joined the Polish Resistance movement. 

He went under cover to the Warsaw Ghetto, where his task was to gather infomation and, above all, observe the Nazis’ crimes against the ghetto inhabitants. After the visit, he reported: 

There was a stench. The streets were untidy. Gutters filled with fecal matter. And corpses. Naked corpses (…) I saw a child dying in front of my eyes and an old man dying.

The next place Karski saw from the inside was the transit camp, which was the actual Jewish ghetto in Izbica, close to Lublin, in Poland. The camp was guarded by Ukrainian guards, and Karski disguised himself as one of them. He witnessed the loading of Czech Jews into the nearby Belzec death camp. He reported as follows:  

According to military regulations, a freight wagon was designed for eight horses or 40 soldiers (…) The Germans gave the order to pack 130 at a time, but they were still cramming an additional ten (…) From the depths of the wagons came a condemning roar and howling. Karski counted 46 loaded wagons that day.

In 1942, Karski went to the Polish government in London, where he showed the Poles in exile microfilms of his report. In December, the Polish Government issued an official diplomatic note, which was the first appeal from an Allied country calling for the defense of the Jews and informing the public about the Holocaust. 

During the following years of war, Karski met with world leaders to discuss the fate of Jews and Poles under German occupation. He was hosted at the White House by US President Franklin D. Roosevelt. Unfortunately, he was repeatedly faced with disbelief. To prove to the world the Holocaust and the struggle of the Polish Underground State, in 1944 he published the book Secret State, which became a bestseller in the United States. 

In the 1980s, the memory of Karski and his achievements was recognised, and in 1982 he was awarded the title “Righteous Among the Nations”.