Actually, Christopher Simpson, in his book the "Splendid Blond Beast," has a good answer to this question. US corporations were making deals with the Nazis prior to the war and members of the State Department were fundemental in pushing these deals. Some of the German companies that received US corporate capital were involved in the slave labor of Jews, most of whom were worked to death. When news of the holocaust reached the US military, they refused to threaten the Germans with crimes against humanity because they feared that US pilots would be charged with war crimes in responce. The army air corps were bombing working class neighborhoods in order to "break the will of the people." And Germany let it be known that war crime preceedings would begin if he US persisted with threats of war crimes trials. Also in the book is a detailed discription about how the state department sabataged efforts by Rosevelt to hold war crimes trials after the war. The British counterparts were doing the same thing. As result, the Neuranburg trials let most of those who were guilty get away. Thay included many Nazis who became CIA assets.
US corporations helped rebuild the German war machine. And if you believe Michael Parenti and other sources, a lot of the equipment used in the holocaust were sold to the Nazis by corporations such as Westinghouse. He sights a book called "Trading with The Enemy." And Ken Silverstien had very good artical about Ford motors work in supplying the Nazie war machine with trucks even after the US entered the war. They made money on the deals.
So the US (or at least the wealthy elite, their corporations and the state dept) shoulder some blame.
Brad Hatch