When Fahrenheit 451 begins, the threat of war hangs over the country and its citizens. When Montag is at the firehouse in Part One, for example, there is an important announcement on the radio:
A radio hummed somewhere. ". . . war may be declared any hour. This country stands ready to defend its--"
This announcement is significant because it occurs around the time that Montag feels threatened by the Mechanical Hound. It is as though the prospect of war in his society reflects the conflict in his mind regarding his own happiness and his commitment to being a fireman.
The threat of war continues as the novel progresses. For Mrs. Phelps and Mrs. Bowles, the war is not a time to worry: they anticipate a quick result and believe that their husbands will not die in the fighting since they have never known anybody die in a war. Montag is shocked by their total lack of understanding and this, perhaps, prompts him to read the poem "Dover Beach," a poem that details the harsh realities of war and leaves Mrs. Phelps sobbing "uncontrollably."
It is fitting that war is finally declared at the end of the novel when Montag has agreed to stay with Granger and the other professors. For them, the destruction of the war offers the possibility of a new beginning: an opportunity to rebuild society.