Let’s call the hypothesis that the base rate of major wars hasn’t changed the constant risk hypothesis. The best presentation of this view is in Only the Dead, a book by an IR professor with the glorious name of Bear Braumoeller. He argues that there is no clear trend in the average incidence of several measures of conflict—including uses of force, militarized disputes, all interstate wars, and wars between “politically-relevant dyads”—between 1800 and today.
A quick note on Braumoeller’s analysis:
He’s relying on the Correlates of War (COW) dataset, which is extremely commonly used but (in my opinion) somewhat more problematic than the book indicates. As a result, I don’t think we should give the book’s main finding too much weight.
The COW dataset is meant to record all “militarized disputes” between states since 1816. However, it uses a really strange standard for what counts as a “state.” If I remember correctly, up until WW1, a political entity only qualifies as a “state” if it has a sufficiently high-level diplomatic presence in England or France. As a result, in 1816, there are supposedly only two non-European states: Turkey and the US. If I remember correctly, even an obvious state like China doesn’t get classified as a “state” until after the Opium Wars. The dataset only really becomes properly global sometime in the 20th century century.
This means that Braumoeller is actually comparing (A) the rate of intra-European conflict in the first half of the 19th century and (B) the global rate of interstate conflict in the late 20th century.
This 19th-century-Europe-vs.-20th-century-world comparison is interesting, and suggestive, but isn’t necessarily as informative as we’d want. Europe was almost certainly, by far, the most conflict-free part of the world at the start of the 19th century—so I strongly expect that the actual global rate of conflict in the early 19th century was much higher.
It’s also important that the COW dataset begins in 1816, at the very start of a few-decade period that was—at the time—marvelled over as the most peaceful in all of European history. This period was immediately preceded by two decades of intense warfare involving essentially all the states in Europe.
So, in summary: I think Braumoeller’s analysis would probably show a long-run drop in the rate of conflict if the COW dataset was either properly global or went back slightly further in time. (Which is good news!)
EDIT: Here’s a bit more detail, on the claim that the COW dataset can’t tell us very much about long-run trends in the global rate of interstate conflict.
From the COW documentation, these are the criteria for state membership:
The Correlates of War project includes a state in the international system from 1816-2016 for the following criteria. Prior to 1920, the entity must have had a population greater than 500,000 and have had diplomatic missions at or above the rank of charge d’affaires with Britain and France. After 1920, the entity must be a member of the League of Nations or the United Nations, or have a population greater than 500,000 and receive diplomatic missions from two major powers.
As a result, the dataset starts out assuming that only 23 states existed in 1816. For reference, they’re: Austria-Hungary, Baden, Bavaria, Denmark, France, Germany, Hesse Electoral, Hesse Grand Ducal, Italy, Netherlands, Papal States, Portugal, Russia, Saxony, Two Sicilies, Spain, Sweden, Switzerland, Tuscany, United Kingdom, USA, Wuerttemburg, and Turkey.
An alternative dataset, the International Systems(s) Dataset, instead produces an estimate of 135 states by relaxing the criteria to (a) estimated population over 100,000, (b) “autonomy over a specific territory”, and (c) “sovereignty that is either uncontested or acknowledged by the relevant
international actors.”
So—at least by these alternative standards—the COW dataset starts out considering only a very small portion (<20%) of the international system. We also have reason to believe that this portion of the international system was really unusually peaceful internally, rather than serving as a representative sample.
A quick note on Braumoeller’s analysis:
He’s relying on the Correlates of War (COW) dataset, which is extremely commonly used but (in my opinion) somewhat more problematic than the book indicates. As a result, I don’t think we should give the book’s main finding too much weight.
The COW dataset is meant to record all “militarized disputes” between states since 1816. However, it uses a really strange standard for what counts as a “state.” If I remember correctly, up until WW1, a political entity only qualifies as a “state” if it has a sufficiently high-level diplomatic presence in England or France. As a result, in 1816, there are supposedly only two non-European states: Turkey and the US. If I remember correctly, even an obvious state like China doesn’t get classified as a “state” until after the Opium Wars. The dataset only really becomes properly global sometime in the 20th century century.
This means that Braumoeller is actually comparing (A) the rate of intra-European conflict in the first half of the 19th century and (B) the global rate of interstate conflict in the late 20th century.
This 19th-century-Europe-vs.-20th-century-world comparison is interesting, and suggestive, but isn’t necessarily as informative as we’d want. Europe was almost certainly, by far, the most conflict-free part of the world at the start of the 19th century—so I strongly expect that the actual global rate of conflict in the early 19th century was much higher.
It’s also important that the COW dataset begins in 1816, at the very start of a few-decade period that was—at the time—marvelled over as the most peaceful in all of European history. This period was immediately preceded by two decades of intense warfare involving essentially all the states in Europe.
So, in summary: I think Braumoeller’s analysis would probably show a long-run drop in the rate of conflict if the COW dataset was either properly global or went back slightly further in time. (Which is good news!)
EDIT: Here’s a bit more detail, on the claim that the COW dataset can’t tell us very much about long-run trends in the global rate of interstate conflict.
From the COW documentation, these are the criteria for state membership:
As a result, the dataset starts out assuming that only 23 states existed in 1816. For reference, they’re: Austria-Hungary, Baden, Bavaria, Denmark, France, Germany, Hesse Electoral, Hesse Grand Ducal, Italy, Netherlands, Papal States, Portugal, Russia, Saxony, Two Sicilies, Spain, Sweden, Switzerland, Tuscany, United Kingdom, USA, Wuerttemburg, and Turkey.
An alternative dataset, the International Systems(s) Dataset, instead produces an estimate of 135 states by relaxing the criteria to (a) estimated population over 100,000, (b) “autonomy over a specific territory”, and (c) “sovereignty that is either uncontested or acknowledged by the relevant international actors.”
So—at least by these alternative standards—the COW dataset starts out considering only a very small portion (<20%) of the international system. We also have reason to believe that this portion of the international system was really unusually peaceful internally, rather than serving as a representative sample.