Many lives were spent getting extensive photographic and otherwise documentation of the deathcamps into the hands of the Pope and Roosevelt, years before the camps were overrun by the allies. The story is fairly well known.
My father was a combat pilot in WW2. He told me that intelligence officers told him in summer 1944 of rumours and secret intelligence of the death camps, but that he dismissed them as the crudest allied propaganda imaginable until they were found by allied troops and the red army and the truth came out. I suspect that reaction would probably have been near-universal.
Indeed. Roosevelt and the Pope were given massive evidence by Jewish partisans, in the hopes that they would react appropriately and publicly or secretively, whichever would be most effective. That trust was misplaced: both parties sat on the evidence and their thumbs until the end of Dec. 44, far too late to do much good. Their loud lamentations thereupon, putting them on the side of the angels--except for those of us with a memory unbesmerched by the soporific effects of media.