Please read your military history.
Soldiers who have never fought are less capable in battle. It doesn't really matter how much they train or how many drills are run. Nothing really prepares you for the trauma of watching your friend of many years die by getting his head blown off next to you, or worse, bleeding out while crying for his mother. There are two kinds of people who witness that: those who curl up and pray for it all to stop, and those who pick up their weapons and keep fighting. Generals never know which kind of men they have unless battles weed out the first kind, by death or discharge. And militaries with too many of the first kind lose wars.
Yes, it's horrible. It's gut-wrenching just to think about it. We'd all be better off if we could resolve our conflicts without war. But in the world we live in, there are too many military and paramilitary actors who are not guided by such Enlightened ideals. What will we do if we are not prepared and they come knocking on our door?
Maybe you don't believe that the same irrational forces that first manifest as teenage graffiti, then grow into organized crime, could ever result in organizations the size of militaries. Maybe you don't believe that, in societies which have abolished all adventure, the only adventure left is to abolish that society. That's fine. I respect that. But what if you're wrong? Shouldn't society hedge that bet, considering the cost? Can no such hedge possibly justify its cost?
There's a reasonable discussion to be had about where the line (i.e. cost) should be drawn, and what shape the line should take. But the opinion that it is worth it to pay some cost should be relatively uncontroversial. And if you understand that society should pay some cost to maintain an effective military, you ultimately understand that, until the diplomats can succeed at building a genuine global peace, war is inevitable.