Young adult humans need quests. In the past, leaders could satisfy their need for pawns and the young adult's need to pursue some glorious aim by putting the adventurous youth into armies.
Humans seem to have a desire to die colorfully in conflicts they imbue with deep significance. It beats rotting in some retirement home.
Humanity made a mistake by taking these wars too seriously and making the killing machinery too efficient. Getting blown to smithereens by an impersonal bomb or cannon shell lacks the fantasy-game satisfaction of flowing swordplay, or even just stalking each other with firearms through a forest or cityscape.
We need to go back to wars of personal combat, where the dance means more than the body count. We can have both war and peace if we acknowledge that war is really just another hobby. Medical technology has advanced very well, forced to treat the grossly traumatic injuries inflicted by massively destructive weapons. Imagine how much fun a war with more primitive weapons will be now. You can get hacked, stabbed and stuck with arrows and then have a state-of-the-art helicopter evacuate you to a modern medical facility. Some people would die, but more would live. And you'd know at the end of each work day that you rose or fell based on your own skills, not some random insult like a land mine or a giant bomb.
Risk of death, and loss of comrades is necessary to keep the quest real.
The organizers of wars could decide what the stakes would be, just as the organizers of charity sporting events decide what cause gets the income from the entry fees. In fact, we could even have benefit wars that really do donate the proceeds to a charity.