Concerning Bradbury's Fahrenheit 451, I don't think the first thirty pages have an impending war as a backdrop. Unless I'm missing something, the only association with anything dealing with war is the sound of jet planes flying over Montag's house. The possibility of war isn't even mentioned until page 32 of my copy. Bradbury writes:
A radio hummed somewhere. "...war may be declared any hour. This country stands ready to defend its..."
Then some jets fly overhead again. That's it. Montag's wife's suicide attempt is key in the first thirty pages, as is Montag's meeting Clarisse. We also know he has something hidden in his home. And something is up with the hound at the station. But I don't see the idea of impending war even surface in the opening of the novel, and the idea certainly wouldn't merit your word, "entire." Again, I could be missing something, but I'm sure other editors will point it out if I am.
Throughout the entire book, there are hints at a possible war. It is never part of the main conflict and storyline of Montag's evolving character and feelings, but rather a backdrop for the main action. The first reference we get to a possible war is after Montag walks into his house and discovers Mildred has tried to kill herself. As he stands there, Bradbury writes,
"the sky over the house screamed...the jet bombers going over, going over, going over, one two, one two, one two, six of them, nine of them, twelve of them."
There are a couple important things to note about this quote. The first is that there are so many jet bombers, and Bradbury's repetition, describing them as always going over, seems to imply that war is a threat. Why else would so many bombers be flying around all of the time? Also, Montag is not surprised or startled at the bombers, which means that they were there quite often--it was a regular part of his life. This indicates that he was used to them, implying that they had been vamping up for war for quite some time.
As you keep reading, there are further hints given as to who the war might be between. It is hinted that everyone else in the world hates America because of their high standard of living and lack of morals. That suggests that America is at war with other nations in the world, for differences in ideologies, and because America has become a target for countries that are starving--they blame their hunger on us. Characters in the novel itself will discuss the impact of the war on their personal lives. I hope that those thoughts helped; good luck!