The American soldiers who liberated the Buchenwald and Dachau concentration camps in April 1945 were horrified by what they found.
They already knew of the enemy's brutality: the Soviets had opened Auschwitz and Allied troops had discovered slave labor camps as well as mass graves of partisans.
At Malmedy, Belgium, during the Battle of the Bulge, Germans troops slaughtered 80 American soldiers who had already surrendered, and there wasn't a GI who didn't know it. But organized mass murder? Many of the liberators wouldn't have believed it if they were not seeing it.
Disbelief had been the usual world reaction for years, even though Hitler's 1925 "autobiography" Mein Kampf made it clear he blamed the Jews for Germany's
woes and would punish them if he could. After his rise to power, thousands of Jews fled the Third Reich; perhaps that was how the "cleansing" of Germany would occur. But by early 1941, almost a decade after Dachau opened as a camp for "political prisoners," intercepted German messages, spies and prison camp escapees intimated that something terrible was happening in Germany and the conquered countries of Europe.
One messenger was Polish resistance officer Jan Karski, a Roman Catholic who secretly visited the Warsaw Ghetto and Izbica death camp. He told his story of torture and murder first to the British, and then to President Roosevelt and American Jewish leaders. Most either didn't believe him, or didn't want to. Others claimed it didn't make sense that the practical Germans would kill off skilled laborers in the middle of a war. Others, especially isolationists and anti-Semites, thought Europe's Jewish community was trying to lure America into the war with fabricated horror stories. (See "The Isolationist Movement" and "American Anti-Semitism.")
After an alleged "conspiracy of silence," American newspapers finally began running eyewitness accounts of deportation and extermination. But by the time Germany's atrocities were acknowledged, Allied leaders argued all they could do to help European Jewry was win the war quickly. Roosevelt refused to bomb Auschwitz or the train tracks leading to it, for instance, because he claimed the bombers were more valuable to the entire war effort elsewhere (probably true), Auschwitz was outside bombing range (demonstrably false), and many Auschwitz prisoners might die in such a bombing raid.
Most of them died anyway.