I think it is also worth mentioning that apathy may be just one way to phrase it. For instance, apathy assumes that there is a choice to be made, and the person or people in question are too lazy to make it. In Fahrenheit, I sense something similar to 1984; people are so obsessed with something (in Fahrenheit, it's entertainment, in 1984, it's the party line) that all else loses value. Do Montag and his wife have sex? Montag saves her, so he must care for her...but what else do we see that says that he cares?
It's possible that war is kept in the background, purposely hidden, until it can't be anymore. In the case of Fahrenheit, that's when the bombs dropped.
I agree with the analysis here at eNotes regarding war. The author of this article argues that the people in Montag's society have become so apathetic and passive in all things that not even war can stir them from their near zombie-like existence.
Here is the excerpt from eNotes. You can find out more about this theme and others, as well as other literary elements, by visitng the link below.
"By portraying many characters as passive figures who never even wonder about their lot in life, Fahrenheit 451 seems to imply that apathy is a very important element in the decline of Montag's society. Millie is content to receive whatever "entertainment" that comes from her television, unable to distinguish between programs that are numbing in their sameness. She has no real concept of what the coming war might mean to her—she only worries it might interrupt her precious television programs—and her friends are similarly unconcerned. Montag's colleagues laugh at him when he wonders aloud about the history of the firemen, and are satisfied with the reasons provided in their handbooks The only action these characters take is to maintain the status quo—the way things are. In contrast, Clarisse, Montag, and Faber are individuals who wonder about their world and, in the case of Montag and Faber, are able to make attempts to change things. Even the book people who live outside of society are eventually able to take action, for after the destruction of the city it is implied they are the ones who will help rebuild the world. "