History happens largely as a result of what people do. Certain people happen, and create a new reality. Certain people cause things to happen.
An example of people who happen would be a person who is born into a certain family, such as a royal family. This person did not cause anything on purpose. It can also be a person who acts by mistake, and changes the course of history. Another example might be Oppenheimer and the atom bomb. Sometimes you don’t really know what effect your actions will have on history.
Other people intend to change history. Adolf Hitler is an example. He sought out to change history, and he did so. He might not have left he legacy he intended, but he certainly left his mark on the world.