What is it good for? But actually?

I haven't read it myself, but based on this summary, Azar Gat's "War in Human Civilization" sounds like it could be helpful for answering your question:

Since I have complained quite a bit about ‘megahistories‘ lately, it seems only fit to offer a proper macro-history that I think it done responsibly. Gat’s book is not a history of battles, but of war in the broad sense. He thesis starts with the questions why humans do war (beginning with the motives of food and sex and then moving to more complex motives which derive from those) and then based on those motivations, the assessed how war has shaped humans.

Perhaps the most explosive of these arguments, but one I find very convincing, is his argument that military mortality in the deep past of human history when we all lived as hunter-gatherers was high enough, for long enough that it exerted evolutionary pressure on the emergence of anatomically modern humans; that is, stated bluntly, humans are evolved for war. This argument, which emerges in his first chapter, is a point of fierce debate among archaeologists and anthropologists (it is rooted in rival conceptions of human nature, after all) but I think Gat has the right of it, and I recall reporting when I first read the book that I wished I could ‘frame Gat’s first chapter,’ a position I still hold.

Subsequently, he sees the rise of the state as a consequence of that human propensity for war, in an argument that will not be entirely unfamiliar to blog readers, as I used it as part of the basis of some of the Fremen Mirage. At the end, he concludes that the human propensity for war has become maladaptive, due to the rising power of human productivity (meaning the war is no longer the best way to get resources as compared to industry and trade) and the rising destructiveness of war (meaning that the costs of war outweigh the gains). Consequently, as Gat poses it, the question is if we can hold off on destroying ourselves (with nuclear weapons) as our genetic programming would suggest long enough for evolution (either social or genetic) to catch up to our sudden, newfound destructive power.

I especially recommend Gat to anyone who has read, or is considering reading Steven Pinker’s The Better Angels of Our Nature, a book I am continually asked if I have read. Quite frankly, I think Gat simply has the better, more rigorous form of Pinker’s argument (which naturally also means somewhat different, more reserved conclusions).

Bronze Age war (as per James Scott) was primarily war for captives, because the Bronze Age model was kings ruling agricultural dystopias amidst virgin land where people could easily escape and become hunter-gatherers. The laborers would gradually escape, the country would gradually become less populated, and the king would declare war on a neighboring region to steal their people to use as serfs or slaves.

Iron Age to Industrial Age war (as per Peter Turchin) was primarily war for land, because of Malthus. Until the Industrial Revolution, you needed a certain amount of land to support a unit of population. Population was constantly increasing, land wasn't, and so every so often population would outstrip land, everyone would be starving and unhappy, and something would restore the situation to equilibrium. Absent any other action, that would be some sort of awful civil war or protracted anarchy where people competed for limited resources - aided by wages being very low (so they could hire soldiers easily) and people being very angry (so becoming a pretender and raising an army against the current king was a popular move). Kings' best way to forestall this disaster was to preemptively declare war against a foreign enemy. If they won, they could steal the enemy's land, which resolved the land/population imbalance and fed the excess population. If they lost, then (to be cynical about it), they still eliminated their excess population and successfully resolved the imbalance.

The other comments are great and address some of the macro issues, but I feel like there's something to be addressed here about the individual decision to fight, and even the individual's willingness to fight, since although history is full of people who unwillingly fought as soldiers, it's also full of people who chose on purpose to go to war and risk their lives on a gamble that gave better than even odds of their dying.

The modern world is very different than the world of the past. On a number of dimensions, our lives are worth more to us now than in the past. That is, I'm not saying non-modern lives are worth less than modern ones in some essential sense, but that the person living in the past likely valued their own life in various ways less than we do. The negative read on this is that we are more narcissistic and think our lives are more worth living than people were in the past.

Some dimensions along which we more value our lives, i.e. these things are more true for us than they were for people in the past:

• we live in greater comfort
  • it was often a painful struggle just to stay alive in the past with no end in sight
• we are more likely to be able to achieve more of our desires
  • most past people were constrained by their physical or political environment
• we can have greater impact through our lives than our deaths
  • for many past people the most impactful thing they might be able to do is protect their kin's land by fighting to the death
• we have greater economic opportunity
  • for past people, their existence was a larger economic burden due to a subsistence level existence, so they might reasonable see good economic opportunity in fighting and dying if that would benefit their kin
• we can better support our kin alive than dead
  • labor in the past didn't produce large returns, often just barely exceeding the level needed to support a few people at subsistence levels, so dying fighting involved less loss of output for the family group
• we can live more years
  • life expectancy in the past was shorter due to disease, malnourishment, and war coming to your doorstep even if you didn't choose to go fight
• our lives have more happy days
  • life in the past involved a lot of difficulty that made the average day simply less full of happiness and more full of pain
• we are more individualistic
  • a big thing we can forget is that in the past most humans were situated in extended kin groups and those kin groups were the primary unit of social organization, not the individual, and so a person might reasonably identify more with the needs of their kin group than their personal needs
• we are more atheistic
  • past people more expected an afterlife, possibly benefitted specifically by dying in battle

Additionally, inheritance practices tended to produce lots of male children who would inherit nothing, so for them the only avenue to prosperity (enough resources to marry and have a family) was often soldiering.

This all means we have a much harder tradeoff to make than ancient people did when it comes to choosing to fight.

Of course, standard caveats that I'm proposing a statistical model here. I have no doubt you can find exceptions to this story all over the place. What I'm proposing is that, on average, more people values their lives more now than in the past, and this accounts for our lower willingness to go to war. The incentives have changed, and dying fighting is less appealing on the margin.

One of the forces present in society is people striving to not just meet some moral standard, but be seen as more moral than others. This is often present both in demonstrating virtue and demonstrating the absence of vice. ["Let me show you how not racist I am!"] For much of human history, courage and strength have been important virtues, and cowardice and weakness important vices.

In England during World War I, as thousands were dying pointlessly in the trenches, pretty girls went around handing white feathers — a symbol of cowardice — to men who weren’t in uniform. [src]

Participation in and advocacy for war are often seen as evidence against personal cowardice and for personal bravery. (The slur "chickenhawk" deflates advocacy without participation, by separating out hollow signaling from substantiated signaling.) Like Kaj, my sense is that people have a baked-in sense of how good war is that's more tuned to our long evolutionary history than the recent, present, or future bits.

This varies a lot based on the context, as you might expect. It is worth pointing out that a lot of the details which speak to your confusions are filtered out just because of the format in which you are consuming history; popular histories necessarily dispense with nuanced details behind popular opinion or decisions of rulers/governments/etc; they spare none at all for the individual soldiers. It is also worth pointing out that this is not even a little bit settled of a question; it doesn't even have rigorous schools of thought - we (the civilizational we) are only in the process of clarifying this now.

That being said, I claim it is because of local incentives.

The simplest one is brutally straightforward: most of history's soldiers were not volunteers but conscripts. In pre-modern times the bargain was this: come with the army or die immediately. Going to war, for the group of men who did, was mostly a matter of sparing their own lives and the lives of their village from their own army. In modern times we have refined this process considerably, but fundamentally it remains a matter of a highly probable harm up front versus a distant and uncertain harm in the future.

Volunteers are more nuanced, but they still act according to local incentives. Most of time it is because the army offers more reliable food, shelter, and payment than they would get by staying where they are; maybe dying of dysentery on the march isn't as bad as definitely freezing or starving next winter. Since the development of the professional military the army is considered a career like any other, and gets put down next to doctor or engineer for officers, and welder or electrician for enlisted.

Volunteers who join for moral reasons (like me) can probably still mostly be chunked as responding to local incentives, where local incentives includes social incentives. A lot of people in this group do it because it is a family tradition, or because it is highly respected in the area where they are from. That being said, this is the group where the explanation is weakest, and I have to start wondering about larger social incentives, and all the sticky wickets that come with those.

In terms of the decision-makers for whether there even is war, I like the post Power Buys You Distance From the Crime [LW · GW] for modern, non-war corollaries.

In terms of what this feels like from the inside, I can offer a first-person perspective [LW · GW].

I know this is kind of a fraught and ineffable subject, so I'll say explicitly that questions are welcome even if they are about my personal experience. If there is discomfort with asking those questions in public, I can answer via PM.

I agree with Kaj; humans have evolved for war. Also, human culture has evolved for war. As a result, war is glorious and appealing, at least to many humans. (Think about how a huge fraction of video games involve fighting. Fighting is fun! Fighting is cool!) I remember reading a quote from a young soldier in Iraq: "War is better than sex!" On the less juvenile end of the spectrum, war is an adventure, and adventure is appealing. War is also a competition, and competition is appealing. War also is a great opportunity to demonstrate virtue, which means that lots of heroes and heroic stories happen in war, which means that when people look for role-models or fantasize about achieving status and glory, war features prominently.

how has there been so much energy for going to war?

I have this question about 90% of human activity.  How is there so much energy for going to bars?  How is there so much energy for watching, analyzing, and endlessly discussing sportsball? 

To be clear, I’m not confused here about people fighting in defense of things they care a lot about

 think a lot can be explained by acknowledging that some/most people care a lot about things that don't make sense to you or me.  And this gap gets bigger with wider divergence in context - readers of this site are at a historically and globally extreme point of wealth, comfort, and breadth of options if we don't want to do what everyone around us expects.

Combine this with a bit of crappy evpsych about social drives of excess young men, and much of the mystery dissolves.

Hm, I notice some confusion here too. It does seem odd that throughout history people would willingly do something so aversive.

My thought is that it's a prioritization of social status over physical pain. Maybe similar to how if you're on a sports team and your coach tells you to run, you do it even if you're 100% totally exhausted. It would be too embarrassing not to, like this scene from Miracle. But the presence of such analogs don't really resolve much confusion. I still am left confused about why people do it in the context of sports.

Generally I am quite wary with explanations of evolutionary psychology, but I think a good point can be made that going to war oversees is very similar to going out to hunt mammoth for the tribe: a dangerous travel-adventure to kill things to help the tribe. I suppose people with such tendencies were more likely to reproduce.

Is the question why tribes go to war, or or why individuals do?

The first question isn't hard to answer. For most of history, there weren't other means of conflict resolution , there weren't other means of economic expansion , and it was often a matter of life and death.

The hard problem seems to be why individuals volunteer for wars that aren't obviously defensive. Humans..well, males mostly ... have evolved aggression , comradeship , loyalty obedience and so on, which enable warfare. But unfocussed aggression is a nett negative where it's not needed. It would be better to have a switch, and we have switches, because we have evolved to be cultural and symbolic.

The objective facts barely matter. People will readily fight over symbolic slights or ideological principles. (Jared Diamond sees the justification of war as the main function of state religion).

I was very influenced in my thinking about war and the long human legacy about tribal conflict by Xonophon's Anabasis. It is seven books (each much shorter than an SSC post) which are kind of a diary of about soldiers' real experiences.