When did the knowledge about the holocaust by the nazis break into general knowledge around the world?

I’ve read lately several books in which at least a part of the story takes place during the late 1930’s or early 1940’s. Several of them have discussed about the holocaust as a reason to go to war with Germany. However, I have had the understanding that the topic was not widely discussed by the public before the war or during the early war years. Thus I’ve started to wonder if the authors are reflecting their later knowledge of the atrocities to explain wrongly the motivation of their main characters to advocate entering the war.

As I am most definetely not an expert on this area, it would be interesting to know more about it.