It probably won’t be a big surprise that this an extremely contentious issue. It’s only made more contentious by the fact that we’re not 100% sure who knew what about the Holocaust and when, although we can consider that, after a certain point, the western Allies undoubtedly knew that Jews were being exterminated in Europe. Since the end of the war, two “sides” have emerged, with one side slightly better supported but the other side making important contributions to the debate.

First, remember that wholesale extermination of Jews began in the summer of 1941 in the USSR. The Soviets were not crazy about their inability to protect their own population getting out, but we also know that British intelligence at Bletchley Park learned about these mass shootings in the fall and early winter of 1941 owing to the breaking of the Enigma code. The United States entered the war shortly thereafter, but neither the UK nor the US had the ability to intervene in the USSR at that point.

The important turning point is 1942. Importantly, of the 5-6 million Jews killed in the Holocaust, more were alive than dead at the beginning of 1942, while the opposite was true at the end of the year. Intervention in the extermination of Polish Jewry during that year would have made the most difference.

Several factors stood in the way, however. First, information about the extermination program came out in small amounts, and there was extreme caution taken in believing reports of atrocities because, in the previous war, many reports of German atrocities had proved untrue. Second, as reliable reports began to accrue, there was still consistent disbelief, including among people we might most expect to be convinced. For example, President Roosevelt asked his close advisor Felix Frankfurter if he thought the reports could be true, and Frankfurter, who had grown up in Austria, said they couldn’t be. Finally, once there was little doubt left about the truth of the exterminations, the front lines made an attack on the actual locations nearly impossible. It would have been guaranteed at the time — during which ten percent of bomber pilots were already being killed — that the bombers attempting to approach the targets in question would be shot down. The final consideration here was whether the home fronts in the US and UK would accept the idea of putting soldiers at such risk to save Jews. Roosevelt and Churchill had had to overcome the propaganda of Germany and its allies and supporters that WWII was a “Jewish war.” It would have been difficult to continue denying the charge if raids on Polish death camps had occurred given the significant levels of antisemitism in both countries.

By the time it was actually possible to bomb death camps, the only one left within range of US and UK bombers was Auschwitz, and in 1944, the remaining largest Jewish population under German control — that in Hungary — would be sent there. Here, the case for bombing the rail lines to the camp is most difficult to refute. US bombers successfully raided Monowitz — a subcamp of Auschwitz that housed a synthetic rubber factory — as Jews were being exterminated a few miles away. Here the arguments against bombing the rail line typically focus on the short term nature of any holdup in killings, since it is assumed the Nazis would have rapidly restored the rail line, as well as the possibility of actually harming the inmates of the camp themselves.

In the last months of the war, Jewish survivors were being used as pawns by people in high positions in Germany seeking a separate peace with the western allies, but that this point, the notion that anything other than unconditional surrender would be acceptable had long passed.

There are several good books on this topic. Peter Novick, in The Holocaust in American Life, summarizes the case against bombing nicely. The standard text making the case for intervention is David Wyman’s The Abandonment of the Jews. Deborah Lipstadt’s Beyond Belief details the difficulty faced in convincing people that the extermination of the Jews was actually happening.