Zinn is a well respected historian. I don't know much about the "bloodbath" (I don't have time to look), but the attack on Hiroshima was unnecessary.
http://www.larouchepub.com/other/200...hiroshima.html
http://www.ihr.org/jhr/v16/v16n3p-4_Weber.html
http://www.nuclearfiles.org/hitimeline/1945.html