For me, one of the more interesting and thought-provoking exercises among historians is the idea of alternative history, or counterfactual history. The idea is a simple one, even if its application can be a bit difficult. The goal is to find key moments of history that with slight changes would lead to massive changes in the actual historical record. For example, the death of a young prince (like the popular and gracious eldest son of James I) could change the dynastic fortunes of the doomed house of Stuart. A single hurricane in Samoa in 1889 shifted the balance of geopolitics in the region, sinking all of the local ships of both the United States and Germany, leading to the division of Samoa between the US and Germany, a divide that still exists to this day between Samoa and American Samoa . Of course, without that storm it is possible that either the US or German fleets would have forced the issue with the possibility of a local war in the region. These sorts of occasions are ripe for counterfactual history because large changes depended on small events that would have been easy to see going another way.
Nonetheless, a major issue with counterfactual history is that there are also second-order changes that can reverse the original changes. This is true even when one considers natural history. For example, one of the most likely climate outcomes if global temperatures continue to increase and ice continues to melt in Greenland is that the Labrador current will eventually cut off the warmth of the Gulf Stream, leading to cooler water temperatures and colder climate in Europe (and North America). Paradoxically, increased warmth could easily lead to lowered temperatures, which is a way that the earth, and history as a whole, shows a greater balance than the race to the extremes that is often modeled by the unwary. Likewise, massive demographic shifts and underlying economic and technological realities tend to show themselves up eventually. One can easily envision a scenario where the Spanish invaders of the Aztec Empire were defeated, but it would be greatly unrealistic to imagine the Aztec Empire leading a counterinvasion of Spain, as the technological and demographic strength was simply missing to undertake such a mission.
In order to do effective counterfactual (or alternative) histories, it is necessary to have a firm knowledge of reality. What possibilities were actually available to the historical actors in question? What options did they really have? What resources did they possess? What realities did they have to confront that constrained their actions? The better we know these factors, the better we are able to see how things could have gone another way. Of course, the better we know the answers to these questions, the better equipped we are as historians to see why particular actions were taken that may appear to us to be counterintuitive, and understanding the bounded rationality and constraints of others better prepares us to face the similar bounded rationality and constraints of historical actors in our present evil times.
Some may dismiss the interest in alternative history as being mere fiction or empty rhetorical exercises, but to the extent that we recognize history as being contingent and not determined, we recognize that human choice matters a great deal. And this choice is not only among leaders, but among common people as well. Demographics are shaped by individual decisions to have children (a sign of future orientation and hope) or not. They are shaped by economic decisions, political decisions (voting), and so on. We are all at least a little responsible for the way the world is and will become by the decisions we make or do not make. Our apathy or dissatisfaction can make our regimes more brittle and vulnerable to overthrow. We have more power than we often think that we possess, and it is imperative to use this power wisely, and to reflect (as the citizens of the late Roman Empire did not) that simply because our existing regimes are corrupt that any replacements are going to be better options for us. Because while things can always get better, they can always get worse. Not all change is improvement.