Change the Course of History

In light of the post on the overuse of the word "tragic," this is in the same vein.
So often the phrase is used "x changed the course of history." Is this really true? More importantly, is it really possible? When an individual changes direction, or course, that person still has an idea of where he is going. But in history, do we know where we are going? Do we know the result? A person or an event can affect history, they can have an impact on it, but can they change the direction? Only if we know what that direction would be without the actor. Put another way, a river runs in a certain direction. A natural phenomena (such as an earthquake) or a man-made phenomena ( a dam) will change the course of the river. Another example is the Dallas White Rock Marathon. Organizers have changed its course several times in recent years. You can't change the course of history without a time machine.