To assume an event that never happened did happen

While writing a paper on 20th century international relations & the Second World War, I was faced with a situation where I had to 'suppose' Hitler never came to power in Germany, and the Weimar Republic never collapsed.

Would there be an academic term to refer to the assumption of an event that never happened, did in fact happen?

One term that conveys this is counterfactual, defined at the Cambridge Dictionary as "about what did not happen but could have happened". It's used to explore alternative histories, etc.

There are other uses for the word, but it does seem to fit here.