The main conflict in Ray Bradbury's 1952 short story, "A Sound of Thunder," is man vs.nature. Eckels lets his fear of the beast he is hunting overtake him, and in so doing, makes careless mistakes. These mistakes have monumental consequences for the whole of human history.
The story takes place in the future and centers on a man named Eckels who hunts big game. He has conquered all the big game in his environment and is drawn to a company called Time Safari, Inc., which specializes in time travel hunting. He pays a large sum of money to hunt a Tyrannosaurus Rex during the Cretaceous period.
Travis, his safari guide, has carefully explained the importance of not disturbing anything in the past. The company takes many precautions to ensure that what they do in the past doesn't alter the future.
Before they take clients on a safari, a guide goes back in time to scout out an animal that was going to die of some natural cause. They calibrate the hunting expedition to within minutes of when the natural death would have occurred so they do not interrupt the natural sequence of events. Travis warns:
"The stomp of your foot, on one mouse, could start an earthquake, the effects of which could shake our earth and destinies down through Time, to their very foundations."
Eckels, having a hard time believing the impact of such small actions, asks further questions, to which Travis replies:
"Crushing certain plants could add up infinitessimally. A little error here would multiply in sixty million years, all out of proportion."
When it comes time to shoot the Tyrannosaurus Rex, Eckels is petrified with fear. Travis becomes impatient with him and tells him to return to the time machine. On his way back, Eckels accidentally steps off the anti-gravity metal path that has been built by the company to ensure that not even a blade of grass is harmed during the expeditions. Travis sends Eckels back to the carcass to remove the bullets, which must not be left in the past. He feels that this will teach Eckels a lesson about signing up for hunts he's not capable of completing. On the way back, Eckels tramples a butterfly.
Upon his return to 2065, he notices the company sign has changed. The lettering is different. He begins to fear that Travis' warnings have been correct.
"Eckels felt himself fall into the chair. He fumbled crazily at the thick slime on his boots. He held up a clod of dirt, trembling. 'No, it can't be, not a little thing like that! No!' Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful, and very dead."
Horrified, Eckels asked the man at the front desk who won the presidential election. Instead of answering President Keith, as he had when Eckels went on the safari, he answers Deutscher, his opponent with dictatorial beliefs. Eckels realizes that when he left the path, he altered the course of human history in exponential ways.
Though there is a minor conflict of man vs. man with Eckels and Travis, the main conflict is man vs. nature. Eckels alters the course of history by damaging an insect, a butterfly. This disruption to the ecosystem sets in motion a chain reaction that alters everything in the known world in a negative way.