This is a very interesting article that discusses whether or not the dropping of two nuclear devices on Japan really was 'necessary' and whether they really were responsible for the Japanese surrender:
http://www.foreignpolicy.com/articles/2013/05/29/the_bomb_didnt_beat_japan_nuclear_world_war_ii
http://www.foreignpolicy.com/articles/2013/05/29/the_bomb_didnt_beat_japan_nuclear_world_war_ii