You have valid points with your abortion point in that it's very often about perception of right and wrong. But it looks like the U.S. at least didn't "know" until a year after they entered the war[1], but even when they did, it appears that they had reservations about acting on it[2] (ie, do you bomb Auschwitz and kill the current prisoners to prevent future prisoners from being gassed there?)

[1] http://www.ushmm.org/research/library/faq/details.php?lang=e...

[2] http://www.ushmm.org/wlc/en/article.php?ModuleId=10005182