A good example of foreshadowing comes as Montag returns home to find his wife, Mildred, overdosed on sleeping pills. This is so far out of his perception of the world that he is shocked into immobility; he can't think of what has happened, why it has happened, or what to do. Then the jets pass overhead:
As he stood there the sky over the house screamed. There was a tremendous ripping sound... The jet-bombs going over, going over, going over, one two, one two, one two, six of them, nine of them, twelve of them, one and one and one and another and another and another...
(Bradbury, Fahrenheit 451, Google Books)
Throughout the book, Montag hears mentions of a possible war on the television and radio, but no one seems interested in talking about it or doing anything substantial to avert it. Sure enough, at the end of the book, jets from the opposing side of the war pass overhead and bomb the city into rubble; it has been clear from the beginning that a war was on the horizon, but the people in the city are so mentally drugged from years of meaningless television -- not to mention the government intentionally keeping them ignorant -- that they have no interest in changing their own lives. The war is the ultimate repudiation of the censored, controlled culture; Montag even imagines Mildred in her last moments, trying to connect with the images on a television screen, without the capabilities to see or even understand larger issues.