My MA adviser always said that the purpose of comparative history is not to discuss the alikeness of two events or historical actors, but the difference. In other words, the purpose is to help define or understand a category.

The best example that I can think of here is Working Toward the Fuhrer by Ian Kershaw. In it, Kershaw begins by discussing the nature of dictatorships, specifically those of Stalin and Hitler. He notes that while both are dictators, the way that each ruled and controlled their respective countries were quite different. From this point, Kershaw explored Hitler's dictatorship and sought to determine how it functioned. Thus, the point of comparison served to demonstrate the complexity of dictatorship and the need to differentiate among dictator's methods of rule.

Tagging along with the others who have said the focus on the difference is the key, because it casts the specifics of each particular case into sharp relief:

In medieval, some of the best comparative studies I can think of don't compare events. They compare contexts--the Italian church and church life vs the English church and church life; bureaucracy and violence across Europe; English vs German street literature. That seems to me like a much more solid base for drawing contrasts, given how contingent events are.

I've realized that, while there are certainly similarities between the War on Terrorism and those other conflicts, I'm not too sure that those similarities are inherently significant. Do they actually tell us anything substantive about either of the conflicts under consideration?

I think that the similarities tell us relatively little. Looking at many events and seeing the similarities lets you guess at general principles, but these are often quite general, and already known. Looking at only two events, the similarities say little.

The differences tell us more. Differences invite the question "Why?", and looking for answers to that question is where we can learn something interesting.

My own critique of comparative history is that it doesn't allow for any analysis of development over time. Historians essentially pluck two events out of history to compare and contrast, and in doing so they decontextualize both events.

The context matters. Without the context, how can one answer the interesting "why?" above?

A lot of comparative history is done very poorly. The decontextualisation you note above is sometimes the cause. Sometimes, it's deliberately done poorly, to push a pre-determined idea. This doesn't help the reputation of comparative history.

There can be value to the practice, but the important thing is to realize exactly what you point out, that taking events out of their proper context robs the analysis of much of their value. However, I know in military history or economic history, the ability to compare past events within the limited framework of that discipline is useful (and even necessary) for analysis.

History is a science where we cannot re-run the experiment with different variables to prove a hypothesis, all we have are our records of what happened. In that case, when it comes to application of theory or lessons learned, historians in theory-focused sub-disciplines lean heavily on specific examples. In this context, comparison can be useful. For military history, while weapons and sides may change, many tactical principles are relatively universal - comparing how those tactics did and did not work in historical context can still be informative to the present, at least in a limited degree. Economic history too often examines significant events through its own lens, looking for causes and effects, seeing how a given theory fits (or does not fit) what happened in past events.

Part of the problem is that comparative history is reflexive; people generally understand the old saw that "those who don't learn history are doomed to repeat it" - implicitly suggesting that you can compare past events and conditions with current or future ones. So you get a lot of amateur and armchair historians who make poor comparisons - the nadir of which is saying that "X person I disagree with is just like Hitler!" - and who don't take into account the limitations of taking historical events and individuals out of context.

One area (as you might expect, connected to my flair) in which comparative history seems to me to be of some importance is the history of ideas. Let's take my favorite philosopher, Rene Descartes, as an example.

There is a traditional narrative in the history of philosophy according to which Descartes marks a sharp break with the past. It is from this narrative that you get characterizations of Descartes as the father of modern philosophy, one of the fathers of modern science, and a key turning point in the history of mathematics.

Now this characterization is not wrong per se; there is of course quite a bit to it. Nevertheless, it is fruitful to compare Descartes those he is often considered to be radically breaking from as well as those who will follow in his footsteps.

Here is an example. Prior to Descartes, you (very roughly) get the Scholastics talking about causation as (i) a necessary connection between two causal relata, (ii) the standard four causes of Aristotle, and (iii) causation as more than merely descriptive of regularities in nature. It is an interesting and informative question to ask which of these theses are accepted by various thinkers supporting or reacting against Scholasticism. One short list might read:

• Francisco Suarez: Accepts (i), (ii), and (iii)

• Descartes: Accepts (i) and (iii), but only partially accepts (ii).

• Hume: Uses (i) for a reductio ad absurdum of (iii). Ultimately, it starts to feel like the subject has been changed more than anything else, as 'cause' is now little more than what we, as a matter of psychological fact, believe about two events that are constantly conjoined in our actual experience, rather than a genuine metaphysical relation.

So by comparing the thought of Descartes with a relatively recent representative Scholastic (Suarez) and one of his fellow "moderns" (Hume) we get the somewhat surprising result that, on this issue at least, Descartes looks a lot closer to the Scholastics. This is interesting as it challenges this traditional narrative of Descartes as a clean break with the past, announcing (almost fully formed) the "modern" understanding of the world.

Now this example is admittedly way, way too simplified to be all that interesting in its own right. But hopefully it suffices to show, at least, why folks interested in the history of philosophy or the history of ideas think comparative history like this worthwhile.

When it comes to widening ones viewpoint it can be useful.

For example one of the best works regarding Cahokia reworks the site using the language of Eastern Asian tributary states. Similarly, when looking at economic history ethnographic examples from around the world are frequently used to better understand non-monied economies because the average reader is often at a disadvantage to understanding these phenomena.