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:
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.
I didn’t learn about history very well prior to my thirties somehow, but lately I’ve been variously trying to rectify this. Lately I’ve been reading Howard Zinn’s People’s History of the United States, listening to Steven Pinker’s the Better Angels of Our Nature, watching Ken Burns and Lynn Novick’s documentary about the Vietnam War and watching Oversimplified history videos on YouTube (which I find too lighthearted for the subject matter, but if you want to squeeze extra history learning in your leisure and dessert time, compromises can be worth it.)
There is a basic feature of all this that I’m perpetually confused about: how has there been so much energy for going to war?
It’s hard to explain my confusion, because in each particular case, there might be plenty of plausible motives given–someone wants ‘power’, or to ‘reunite their country’, or there is some customary enemy, or that enemy might attack them otherwise–but overall, it seems like the kind of thing people should be extremely averse to, such that even if there were plausibly good justifications, they wouldn’t just win out constantly, other justifications for not doing the thing would usually be found. Like, there are great reasons for writing epic treatises on abstract topics, but somehow, most people find that they don’t get around to it. I expect going to some huge effort to travel overseas and die in the mud to be more like that, intuitively.
To be clear, I’m not confused here about people fighting in defense of things they care a lot about—joining the army when their country is under attack, or joining the Allies in WWII. And I’m not confused by people who are forced to fight, by conscription or desperate need of money. It’s just that across these various sources on history, I haven’t seen much comprehensible-to-me explanation of what’s going on in the minds of the people who volunteer to go to war (or take part in smaller dangerous violence) when the stakes aren’t already at the life or death level for them.
I am also not criticizing the people whose motives I am confused by–I’m confident that I’m missing things.
It’s like if I woke up tomorrow to find that half the country was volunteering to cut off their little finger for charity, I’d be pretty surprised. And if upon inquiring, each person had something to say—about how it was a good charity, or how suffering is brave and valiant, or how their Dad did it already, or how they were being emotionally manipulated by someone else who wanted it to happen, or they how wanted to be part of something—each one might not be that unlikely, but I’d still feel overall super confused, at a high level, at there being enough total energy behind this, given that it’s a pretty costly thing to do.
At first glance, the historical people heading off to war don’t feel surprising. But I feel like this is because it is taken for granted as what historical people do. Just as in stories about Christmas, it is taken for granted that Santa Clause will make and distribute billions of toys, because that’s what he does, even though his motives are actually fairly opaque. But historical people presumably had internal lives that would be recognizable to me. What did it look like from the inside, to hear that WWI was starting, and hurry to sign up? Or to volunteer for the French military in time to fight to maintain French control in Vietnam, in the First Indochina War, that preceded the Vietnam War?
I’d feel less surprised in a world where deadly conflict was more like cannibalism is in our world. Where yes, technically humans are edible, so if you are hungry enough you can eat them, but it is extremely rare for it to get to that, because nobody wants to be on any side of it, and they have very strong and consistent feelings about that, and if anyone really wanted to eat thousands or millions of people, say to bolster their personal or group power, it would be prohibitively expensive in terms of money or social capital to overcome the universal distaste for this idea.