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.

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pc86 9 hours ago | parent [-]

> 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?

I mean the assumption is that if the first missile hit the building, the second missile would have been fired at something else, right? Still seems worth it at face value especially if there's enough time between the two missiles that there aren't people in the building anymore.

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wat10000 7 hours ago | parent [-]

My assumption would be that the attacker builds missiles based on the defenses they want to defeat. If you have no defenses, maybe the defender builds 1,000 missiles. If you have 1,000 interceptors with 100% accuracy, then maybe the defender builds 2,000 missiles.

This is why the superpowers mostly scrapped their ICBM defenses in the 70s. The technology worked fine. It's totally doable with 1970s technology if you're willing to put nuclear warheads on the interceptors. But for every ICBM interceptor you built, the other side could build another ICBM for the same cost or less. And you need more than one interceptor per ICBM since they can fail and the each interceptor only covers a small area. Add in multiple warheads on a single missile and decoys and suddenly you might need 10x or more. So the USA gave up on the idea of covering the entire country with interceptors, deployed a few interceptors to protect some missile silos, then shut it down after less than a year. The USSR built out a system to protect Moscow and only Moscow, which is still operational today. However, the British were able to maintain the ability to defeat that system and destroy Moscow with a single submarine, all on their own, never mind what the USA would throw at it.

If you have a certain amount of stuff you can build and you're deciding what to do with that capacity, it's not at all clear that missile interceptors are a good use of that capacity even if you're protecting objects that cost orders of magnitude more than the interceptors cost. It works if you're defending against a far less capable adversary (Israel's Iron Dome against Hamas, USA's GBI system against North Korea) but not with an enemy that's even vaguely close to being a peer.

	▲	bluGill an hour ago \| parent \| next [-]
		That worked in 1970 because there were exactly two players who had incentive to not spend all the money so they agreed to reduce the total ICBMs instead. In the current world there are too many actors - it won't work, they can make thousands of missles. Ukraine has already proven you don't get to control when you are attacked. Thus the only option today is cost reduce defense and produce enough to intercept several thousand per day.
	▲	david_pearce 40 minutes ago \| parent \| prev [-]
		>However, the British were able to maintain the ability to defeat that system and destroy Moscow with a single submarine, all on their own What are you referring to here?