Alternate histories (AH) are interesting. The premise of all AH is that something or some moment happened differently than we know happened. From that premise has grown a whole genre of SFF tales postulating what would have happened if Caesar had not been assassinated or if Columbus had not gotten his ships or if the black plague had not wiped out Europe. Related to that genre is scholars and historians speculating what might have happened if such and such even was changed based on the choices available at that time. In Impossible Histories, Hal Johnson takes the reader on a series of What Ifs through a number of eras in history.
Hal Johnson opens with a philosophical prelude (What does it mean for something not to have happened?) and follows that with 20 What Ifs and ends with s philosophical postlude (How do you make things that did not happen happen?). In each of those 20 What Ifs, Johnson lays out what actually happened and they brings out the change and how that change would have reshaped the world. For example #2 Vikings in North America: What if Leif Erikson had Tarried in Vinland - Johnson discusses why Erikson ends up in Greenland and then Vinland and provides a brief history of what happened in our world. Then he postulates what the outcome would be if the Vikings had stayed longer, brought horses and their diseases with them? Would the Spanish faced armored horseman when they came later? The What Ifs Johnson looks into cover war (World War I, World War II, World War III, Vietnam War), ancient history (Socrates dies, Julian the Apostate, 1st and 3rd Crusade, Rome and the dangers of bathing!, and Ethiopia vs Yemen), individuals (William Morgan and Freemasonry; Sigmund Freud reading Sophocles; Vice President Henry Wallace; Samuel Taylor Coleridge in America; Harriet Beecher Stowe vs Aaron Burr; and Seward assassinated), and odds & ends (British Navy and fresh fruit).
Hal Johnson provides very readable Wat If scenarios that the reader can easily follow with sources that back up his historical renderings. Some of the What Ifs are more plausible than others, but all make for interesting reading.