The condition once stated, the question implied, the answer is implicit: while Hitler systematically massacred the Jews of Europe, the United States and Britain knew his intentions, knew when the colossus of all crematoria at Auschwitz was completed, and until January 1944, did nothing. In doing nothing they actively aided Hitler who proclaimed their disregard as confirmation of his policy; what he inferred Mr. Morse substantiates from State Department records--in the period from August 1942 when the Allies were alerted to the implementation of the ""final solution"" to the creation of the War Refugee Board at the end of 1943 they were immobilized by belief ""that rescue was incompatible with (their) principal war aims."" Even more devastating--and indicative--was their refusal to permit resettlement before incarceration; the simple fact is that the vast numbers of Jews who might have been saved from 1933 onward (when German persecution was authoritatively and repeatedly reported) were not wanted in most of the world, particularly not in the United States, not even twenty thousand children. (Roosevelt helped kill this last proposal, only one of many examples of his long inaction.) By 1944 time had run out for most of the millions but the initiative of the War Rescue Board and its collaborators saved several hundred thousand in little more than a year--evidence that effort was not futile and the only bright passage in the dismal history. To amass and compress this comprehensive record (largely from official archives) was a prodigious undertaking; if it is not widely read it will be less because Mr. Morse's narrative style lacks finesse than because the truth, well supported, is insupportable.