Auschwitz-Birkenau, the largest Nazi concentration and extermination camp, was not directly bombed by Allied forces during World War II for several reasons:

1. Military Priority: The primary objective of the Allies’ bombing campaigns was to target military and industrial sites that would weaken Germany’s ability to wage war. At the time, the main focus was on destroying the enemy’s war machine, transportation networks, oil refineries, and factories producing weapons and materiel. Auschwitz was initially perceived as a labor camp rather than an extermination center, and therefore not a top priority target.
2. Intelligence Limitations: The extent of the Holocaust, particularly the mass killings at Auschwitz and other death camps, was not fully understood by the Allies until much later in the war. Intelligence information regarding the true function and scale of Auschwitz was incomplete and inaccurate. It wasn’t until the spring and summer of 1944, when the Hungarian deportations began, that detailed information started reaching the Allies through various sources, including Jewish organizations and escaped prisoners.
3. Operational Difficulty: Bombing Auschwitz presented operational difficulties. Precision bombing was not as advanced during World War II as it is today, and accurately hitting specific buildings within a concentration camp without causing significant collateral damage to nearby prisoners was extremely challenging. Moreover, the Allies were concerned that bombing might harm prisoners rather than disrupt the killing operations.
4. Diversion of Resources: Diverting bombers to attack Auschwitz would have meant taking planes away from other strategic targets. The Allies believed that defeating Nazi Germany as quickly as possible, including by continuing to press attacks on the German heartland and support the invasion of Europe, was the best way to save lives overall.
5. Alternative Efforts: Efforts were made to bomb nearby railway lines and bridges to hinder the transport of prisoners to the camp. However, these attempts were not specifically focused on disrupting the operation of Auschwitz itself.

It is worth noting that there have been extensive historical debates about whether the Allies could or should have attempted to bomb Auschwitz, considering the moral implications and the technical feasibility. Some historians argue that even if the bombing had taken place, it may not have significantly disrupted the killing operations, given the Nazis’ determination to carry out the Holocaust and their capacity to adapt.