Why do human societies fight wars? This is one of the most basic questions we must try to answer in order to understand foreign policy, but unfortunately it's very difficult to determine if there's a scholarly consensus on this question. While the College of William & Mary's TRIP program has polled thousands of America's international relations scholars on a variety of issues since 2014, their questions focus mostly on currents events rather than core issues in their field.
However, even if we can't determine what most IR scholars think about war, perusing the scholarly literature can at least give us a basic overview of the major theories. In general, the causes of war can be separated into two categories each of which are the domain of different schools of thought in international relations: (1) "realists" focus on competing economic interests & differences in military power as the primary causes of war, whereas (2) "constructivists" focus more on the cultural & psychological causes of war.
This meetup will review some of the major ideological causes of war as theorized by the constructivist school. However, before we do that, it would help to review the realist theories of war...
BRIEF REVIEW OF REALIST THEORIES OF WAR:
Realist theories of war are based on the "bargaining model of war" which states that war occurs when two conditions converge: (1) two or more nations have a grievance, and (2) there's a failure of bargaining to resolve the grievance peacefully. Provided that adversarial nations can correctly gauge each other's military power and come to similar estimates on the chances of either side winning and the projected costs of a war, it's always theoretically better for both sides to simply bargain for a settlement that's less expensive than a war. https://en.wikipedia.org/wiki/Bargaining_model_of_war
However, the bargaining failures that lead to war have several well-known causes: (1) "issue indivisibility" which makes some types of compromise impossible, (2) uncertainty & "information assymetry" which leads to diverging estimates about the risks & costs of a war, and (3) "credible commitment problems" due to fear of the other nation cheating on a treaty or launching a preemptive strike.
A modification to the bargaining model involves accounting not merely for absolute gains but relative gains from war - i.e. countries may be primarily concerned with their international standing & relative power vis-à-vis their neighbors rather than any specific material advantages they could hope to gain from victory in a war. Thus, a state that has static or declining power may decide to start a war with a rising power before they are surpassed. (The idea that a clash between an established power and a rising power is probable or inevitable is known as the "Thucydides Trap".) https://en.m.wikipedia.org/wiki/Graham_T._Allison#Thucydides_Trap
IR scholars posit different conditions which they think yield bargaining conditions & cost-benefit calculations that are less likely to lead to war. Those who believe in "hegemonic stability theory" think one major power acting as the "global policeman" is more stable, while those who believe in the "balance of power theory" think several major powers with about equal power is more stable. https://en.wikipedia.org/wiki/Hegemonic_stability_theory https://en.wikipedia.org/wiki/Balance_of_power_(international_relations)
Bruce Bueno de Mesquita's "selectorate theory" helps explain how the cost/benefit calculation for war varies between autocracies & democracies. Autocrats typically only have to satisfy a small & powerful elite (i.e. top generals, heads of crony industries, religious leaders) while leaders in democracies typically have to satisfy the majority of the electorate. In terms of war, it's easier for an autocrat to sacrifice the common good (i.e. lots of casualties among enlisted men) for concentrated benefits delivered to the elite. All else equal, selectorate theory predicts that autocracies will be more likely to engage in "wars of choice" & "wars of aggression" while democracies are more likely to engage only or mostly in "wars of necessity" & "defensive wars". https://en.wikipedia.org/wiki/Selectorate_theory
"Democratic peace theory" helps explain why democracies rarely go to war against each other. One reason is the wider selectorate which makes leaders more accountable to the public interest, but another reason is that democracies typically have a more open public sphere with news coverage & public opinion polls which creates less uncertainty about a nation's war resolve. One of the major problems with democratic peace theory is that democracies should theoretically be less likely to go to war with both other democracies & autocracies, but in practice they're only less likely to go to war with other democracies. https://en.wikipedia.org/wiki/Democratic_peace_theory
Some scholars have suggested that it's not their form of government that makes democracies less likely to go to war but rather their tendency to adopt free market economic policies. "Capitalist peace theory" explain why countries with free trade are less likely to go to war with one another mostly because it's often easier to trade for resources rather than try to seize them through war & occupation. Also, capitalist countries are typically industrialized, and it's easier for an industrialized nation to grow its economy through innovation which allows more value to be extracted from the existing territory & its populace rather than trying to conquer new territory & new subjects. https://en.wikipedia.org/wiki/Capitalist_peace
Realists often focus on "resource wars" and the idea that the drive to conquer areas with natural resources is a major driver of war. A recent shows that civil wars are much more likely to draw outside intervention if they are in oil-rich nations. https://psmag.com/news/oil-behind-civil-war-intervention
It's possible to reconcile these two views by noting that imperial conquest is associated with early-stage industrialism when labor is still cheap and there's a huge demand for raw materials. Once a country transitions into an advanced industrial economy with a skilled service sector, imperial conquest is no longer profitable and it's cheaper & easier to just negotiate a trade deal to buy resources on the global market. https://foreignpolicy.com/2005/09/09/the-commercial-peace/
The last major realist theory we'll consider is "deterrence theory". "Deterrence" explains the way in which a state can prevent attack by developing its military capabilities to punish aggression. In the pre-nuclear era, this was often achieved through building a large conventional military, but with the advent of nuclear weapons some nations have come to rely on WMDs for deterrence. However, the fear that nuclear weapons create in other nations can lead to proliferation, possibly making nuclear war more likely in the long run. And since the cost of nuclear war is so high that countries are reluctant to use them, this creates the "stability-instability paradox" where nuclear-armed countries are often more willing to engage in minor conflicts. https://en.wikipedia.org/wiki/Deterrence_theory https://en.wikipedia.org/wiki/Stability–instability_paradox
As we'll see in this discussion, constructivist theories of war posit that ideological clashes may play a major role in precipitating or prolonging wars in a way that can't be captured by the cost/benefit calculations in the bargaining model.
