The holocaust and other atrocities were well known during the war, reports were smuggled out of the camps and in to Allied territory as early as 1941.

For instance by Captain Witold Pilecki.

PDF Warning.

There was the Bund Report, and in December 1942 The New Republic published one of the very first accounts of the Holocaust that would reach Western eyes. Aptly titled "The Massacre of the Jews," the piece laid out in minute detail the horrors being perpetrated against the Jews of Europe and indicted the Nazi government, Adolf Hitler, and Joseph Goebbels.

"The Story of a Secret State" by Jan Karski was published in 1944 in America. Karski risked his life with the Polish underground and was involved in missions to the Polish government-in-exile in London. A prisoner of Nazis, and tortured by the SS, he escaped from Poland and in 1943 was sent to London with a hidden microfilm revealing conditions under the Nazis and in particular the relentless persecution of Europe's Jews by the Third Reich.

Karski was smuggled by Jewish underground leaders into the Warsaw ghetto and infiltrated the Belzec death camp dressed as an Estonian guard. He travelled across occupied Europe to England, and eventually to America. Karski personally reported to the Polish prime minister in London, General Sikorski, Britain's foreign secretary, Anthony Eden, the US president, Franklin Roosevelt, and many other prominent figures.

The main Allied justification was of course the Axis invading most of Europe, the war was one of self-defence and the liberation of occupied countries.