In Fahrenheit 451, why does Ray Bradbury set the story with a war looming in the background?
There are several possibilities as to why Bradbury write a story that has war constantly looming in the background. The first is because it is an essential element to the eventual plot of the story. At the end of the novel, that war comes into play in a highly significant way....everything that occurs in the novel up to that point leads to the end, which will require a fully revolutionalized Montag to take his freshly awakened perspective and use it to try to rebuild society better than it was before. For that to occur, society as they knew it needed to be significantly altered. War is the perfect vehicle for such change. So, the war, although in the background for much of the novel, becomes a crucial, front-and-center element of the storyline by the end.
Another reason that Bradbury put war in the background is to highlight some of the problems that existed in their society at the time. The characters in the novel react to war with a cold, chilling indifference; their husbands go to die in the war and they brush it off, and talk about remarrying as if it's trying on a new dress. This highlights the emotional detachment that is so prevalent in their culture, and offset of people choosing to numb themselves with empty entertainment instead of enliven themselves through the deep process of reading. The other detrimental societal impact highlighted by the war is the violence that is inherent in their society; becuase the citizens are so desensitized to violence (through their entertainment), war is a symptom of that. And a last element of their society that war highlights is that there are higher forces at play that are taking advantage of the apathetic masses to attain their own powerful motives. There are powerful people in play, trying to dominate the world through war, and they are able to do so because the masses are so numbed to it that they don't protest.
One last potential reason Bradbury tossed war into the mix is to highlight one of his assertions, which is that materialism and entertainment lead to other vices in society, one of them being a craving for violence and power. Bradbury is very anti-technology, and his book seems to indicate that he feels it strips people of their souls, of their compassion, and of their humanness. War is a result of all of that technology and advancement that often requires a sacrifice of morality and heart.
I hope that those thoughts help to get you started; good luck!