Unfortunately, necessity doesn’t imply possibility. It could simultaneously be true that you must build interceptors to protect yourself, and that you can’t build enough.

It only makes sense to consider the cost of what’s protected if it’s actually protected. If your million-dollar interceptor protects a multi-million-dollar building from a $100 missile, and then that building is hit by a second $100 missile, was it worth it?

That’s the math that has prevented missile defenses from being deployed on a large scale despite being technologically possible for well over half a century now, and despite the fact that a single interceptor might be saving an entire city from a nuclear warhead.

An interceptor costs at least as much as what it intercepts. Take into account miss rates and the cost of defense is a multiple of the cost of offense. Add in the fact that the attacker can concentrate an attack but the defender has to defend everywhere, and multiple warheads on a single missile, and the cost of defense multiplies further.

If defense costs 10x more than offense (a conservative estimate, I’d say) then that means you need to dedicate 10x of your economic capacity to it than your attacker does. If your attacker dedicates more than 10% of what you can put into defense, you lose. Defense can work, but it needs to be against a far weaker enemy. Thats why the most prominent example is Israel defending against neighboring non-state actors. Israel is wealthy enough, and the groups shooting at them are poor enough, that the math works out in the defender’s favor. Iran is a rather different story. And of course defending the US against the likes of Russia and China is a fever dream.