This isn't the world we've built, it's the world we live in. We didn't create this world, and we aren't yet powerful enough to change the nature of reality, so to blame humanity for the necessity of defense is silly. You'll be a lot less dispirted if you accept that the world we live in isn't a human creation and that we still haven't made it out of the struggle to survive.
There's nothing in the nature of reality that forces us to fight each other, unless you mean human nature. But we have a choice in whether we choose to be self-serving, which inevitably leads to conflict, or we choose to love our enemy. Loving your enemy requires dying to yourself, usually metaphorically, but it is an option. Humanity is entirely complicit.
In fact, each person is a microcosm of that dynamic. Each time we sacrifice a little bit of someone else's well-being for our own, we are engaging in a small bit of that warfare. Cut someone off in traffic because you're in a hurry; cheat a little to get ahead; don't clean up your public mess because laziness/rushed. It's all essentially that same dynamic.
>There's nothing in the nature of reality that forces us to fight each other
The reality that we are each capable of disagreeing with each other and having conflicting priorities, combined with the limited resources we have access to, is the nature of reality that forces us to fight each other.
> But we have a choice in whether we choose to be self-serving, which inevitably leads to conflict, or we choose to love our enemy.
Sounds very poetic, but how exactly does this apply to the case of Ukraine being invaded by Russia? Do you propose that the Ukrainians should love the invaders? If that should happen, then the Poles and the Romanians should also love the Russians when their time to be invaded comes?
If people were not pulling the oars, the galley of war would never leave the harbor. People have to remember there are more of them on the oars than there are those who hold the whips, although this is no easy feat when the incentives to paint this simple truth as some desperate impossibility are so high.
It’s all so simple until a missile strikes a hospital full of childern. Then what? Do you think you’ll be praised for your wisdom of oars and helms if you talk to the parents who just lost a child a week ago?