What do you think is meant by the expression the "shot heard 'round the world"?

When two countries where at war they faced each other at the battle of Gettysburg. The two countries decided to make a truce until a random gunshot was fired. At that moment, the two enemies started to kill each other. At the end of the battle, no one knows who fired the first shot.

This phrase has become an idiom or a saying used to describe a event that had a great effect. It originally was used to refer to the Revolutionary war. The first shot in the USA fight against England.