The first part is your description of an actual historical event. You are free to choose any event, as long it is relevant to American History and the time period of the course.
2. The second part is you will change a part of that event and how that affected the outcome. This means an actual change with reason. It isn’t enough to say “slavery did not exist”, I need to know why in your new history it never occurred. For example Hitler did not invade Moscow because he had more accurate intel about the Russian defenders on the European front.