John Keegan "The Second World War" will give you a good documentary source for the reasons to drop the bomb. The best way to argue this is simply put the listeners in the mindset of what was THEN going on. Put it into the context of the finally 1.5 years of the war and the growing casualty lists. Remember also the The Battle of the Bulge aspect. Despite "everyone knowing" the Germans were on the ropes, they launched a surprise counter attack in Dec 1944 that killed an wounded more Americans then any other battle in Europe or North Africa. Kind of a knocked Washington's comfortable assumptions on their butt.
The Revisionists for political and emotional reasons look back and say "Because I know this now, they should of done that then". It is pure crap Methodology. The best way to defeat it is recreate the factual pict based on THEN and weed out all the after the fact Monday morning quarterbacking