What are some of the most interesting "what ifs" in history?

I'm looking for some interesting "what ifs" in history that could be argued, either in academic circles or just in general. The best ones usually come from books because the author is able to argue it in a lot more detail. One of my favorites is from a book on the history of Christianity in Japan (Christianity was banned in the 17th century). The author argues that if the Japanese Christians had been able to make their cause national, rather than a personal faith, then it's entirely possible Christianity would have been able to supplant Shintoism and become the religion of Japan.