Extensive and detailed information on the Holocaust was known to the Allies since 1942 thanks to the detailed testimonies and other accounts of Jewish escapees from the camps and the Warsaw ghetto (Grojanowski Report). The Polish government in exile, based in London immediately had it translated into English and added to another report on the same subject. The Soviet high command was also aware of the killing and sent its own reports to the Allies. In December of 1942 the Joint Declaration of by Members of the United Nations was made public and it condemned the massacres publicly.
However, the Allies also decided not to publicize it too much for several reasons, including that the extent of the killings were so horrendous and unique in their scope and magnitude that they feared public opinion would not believe it and start considering such reports as yellow journalism. They also feared that focusing so exclusively on the issue of the extermination of Jews would alienate segments of the population who were against the Axis powers yet profoundly anti-Semitic.