This evidence was not admissible and did not confirm the holocaust design. War is hell and atrocities occur. Karski was a third party making allegations. In 1942 the USA was still in preparation for the longer war. The UK was on its knees.
To think Karski’s flimsy report would compel all the allied forces to attack Nazi detention camps is imbecilic. The allied effort was only getting started and the strategy to defeat Nazi Germany was carefully plotted given the constraints of logistics, men and critical targets.
The bombings of Dresden and Hiroshima/Nagasaki could be construed as war crimes and some still think they are. Does that then mean that Americans at the time of WWII should have deposed FDR and Truman for crimes against humanity? Could any power on Earth defeated the Americans for these confirmable war crimes? The answer is no. There was no power on Earth that could go up against the US military forces. And the same holds in 1942 for the Nazi war machine. They were powerful and dangerous. The Allies had to pick their targets and pick them well. Every bombing mission counted. The priority was to stop the Nazi war machine first.
The holocaust was a rumor until it was proven a fact when American soldiers finally arrived at the concentration camps and found the bodies stacked 20 feet high. Those brave soldiers took photos, movies and made interviews of survivors and surrounding villagers to understand the full extent of the crime and then and only then was it understood that it was by design from Hitler’s SS.
The new face of Holocaust denial. Karski met FDR, by the way. The "brave" American soldiers (who's saying they weren't brav, and what does anyone's bravery have to do with what Netanyahu is saying?) didn't discover the death camps located in the areas "liberated" by the Soviets.