It's a really good question i never ask myself before, mainly because i though allied weren't aware of the importance of the shoah before the liberation. I did some research and found that they knew about it around 1942 (Richard Breitman). But they didn't seem to understand fully before 1943. In 1942 the BBC said that around 700 000 Jews were killed by the nazi. The 24th march 1944 Roosevelt talk about the fact that the nazi systematically killed civilian without really talking about the jews. The truth was told only after the end of the war. In french: http://lemonde.fr/shoah-les-derniers-temoins-racontent/article/2005/07/18/les-allies-savaient-ils_673523_641295.html Sorry for not answering your question, just some fact.