In the story "A Sound of Thunder," what small cause leads to what enormous effect? W

In this story, the main character, Eckles, and some other
men go way back into the past to go hunting for a Tyrannosarus Rex.  They have to be
extremely careful so as not to do anything that will change history.  For example, they
can only walk on a given path and they have to only kill a dinosaur that they know is
going to die in a moment anyway.

But Eckles panics and runs
off the path.  He apparently steps on a butterfly when he does this.  This causes enough
changes in history that, when they get back, the US has been taken over by a fascist
government.

So small cause = dead butterful; big change =
America becoming fascist.