My teacher made a requirement for an essay that it cannot contain alternate history. For instance, no "if hitler had never lived" or "if the Roman Empire never fell". I want to know why alternate history is an issue when looking at the meaning of an event.
Edit: Wow thanks guys. You went above and beyond what I was hoping for! Imagine if you hadn't shared what would have happened...
Alternate histories can make thrilling, engaging, and intellectual stories. A well-researched tale in which some tweaked events extend Roman rule in Europe 200 years could be a wonderful novel. But that is all it will ever be: stories, tales, and novels.
The idea of an "alternate history" in any setting outside of creative writing suggests that we possess some ability to predict how human history will proceed. That is an outrageous notion. This is not chemistry, where we can take five known chemicals and mix them together in an unstudied mixture and predict what will happen with reasonable accuracy. Assuming a controlled environment, this really only involves 5 variables with which chemists are rather familiar. If history is a science experiment, it is one with billions of unpredictable human "variables" in an even more unpredictable environment. There is no real value to saying what could have happened because it is so impossible to predict.
Because the source for alternate history is your imagination. There's nothing provable or testable, just speculation. While that can be a fun past time, it's not useful for any serious study.