One example of citizens’ fear of the outside world is their burning of books. Another is the attack by the bombers.
In dystopian societies, people fear the outside world. In Montag’s community, the people have protected themselves from their own society by eliminating all trappings of culture: they have eliminated books. Books have been banned, because books are dangerous. Books contain ideas. However, Beatty tells Montag that the banning of books did not start from a central authority. It came from the people. Books fell out of favor gradually as technology fell in.
It didn't come from the Government down. There was no dictum, no declaration, no censorship, to start with, no! Technology, mass exploitation, and minority pressure carried the trick, thank God. (Part I)
The people develop a totalitarian government, where just having books is illegal. This results in citizens turning in each other, and even Mildred turning in her husband. There is also the presence of the Mechanical Hound, which is terrifying. Citizens are hunted down if they break the rules.
Another example of the people’s fear of the world is the bombers. Montag sees bombers in the sky, and there seems to be a constant threat of war.
Is it because we're so rich and the rest of the world's so poor and we just don't care if they are? … the world is starving, but we're well-fed. Is it true, the world works hard and we play? Is that why we're hated so much? (Part II)
The threat of war is real. After Montag leaves, he sees the city burning. The civilization has self-destructed. It is burning, completely destroyed. The outside world that the people fear comes crashing down on them. The outside world is not the only thing that the citizens have to fear. They are destroying themselves.