No, the United States was not morally right to hold off on getting involved in WWII.  However, the US government really did not have any choice due to the overwhelming public sentiment for staying out of the war.  WWII offered a stark moral choice.  It was a war in which (particularly in 1939) democratic countries were fighting against an evil and aggressive regime.  The US did not, of course, know that the Holocaust would happen, but it did know that Hitler was bent on conquering as much land as he could take and it did know that his regime was racist and oppressive.  Therefore, it would have been morally better if the US had gotten involved right away.