So, here's a thread on missile defense, since the U.S. is trumpeting shooting down an ICBM from an AEGIS. This is about why I was okay with SDI in the 80s and think missile defense is now a gigantic waste of money and that "We have a defense!" announcements are a bad idea. /1

When I was working as a consultant on SDI stuff in the 80s, I recall two major assumptions: One is that it would freak the Soviets out. The other is that if it were ever built, it would be stationed above our ICBM fields as a point defense to complicate Soviet strike planning. /2

You can argue about whether "freaking the Soviets out" was a good idea. It almost backfired because it convinced at least some Soviet leaders that we were looking to start WWIII. But it did convince the Soviets that we were determined to win a qualitative competition. /3

Put another way, they were terrified at what would happen if we pumped all that money into military R&D. In later years, some Soviets admitted that they had brought this on themselves. But whatever. That was the old political goal. /4

I didn't know anyone who believed in the Reagan "peace shield" pitch. That was for public consumption. The smart idea was to complicate Soviet first-strike planning, even if just by a hair. A moment of enemy hesitation could be the difference between peace and apocalypse. /5

With the USSR gone, I didn't care what happened to SDI as a program, and I thought it was mostly a waste of money. Bush 41 renamed it and scaled it back to a protection against accidental launches. Again, whatever. SDI served its purpose and it was time to close it up. /6

But no program ever dies. So it just kept getting funded without anyone really thinking too hard about whether defenses are a good idea in terms of actual *deterrence*. Does having a half-assed defense matter? No. I talk about this at length in my book on nukes, NO USE. /7

Mostly, defenses don't matter because no POTUS will rely on them. "We think the enemy's ready to launch. But no sweat, Mr. President, we have a system that's got a 50/50 shot at knocking it down!" Like that's going to matter. "Preempt and take it out now" will be the order. /8

Now, as an "if all else fails, we might get lucky and limit damage" system, great. You want to spend tens of billions on *that*, that's back to the Bush 41 idea. But this "we shall scare our foes with our mighty defenses" tweet is a bad idea. /9 https://twitter.com/USArmsControl/status/1328807174026518533

It's a bad idea for a lot of reasons. First, you can really never test it under battle conditions because, duh. And if you decide to flex your muscles and shoot down and enemy's test - an idea that's been floated - and you miss, you've revealed a lot to your opponent. /10

Second, if you believe that deterrence, rather than warfighting, is the key to peace, then "we'll duke it out with you under our missile shield" is a competing message that suggests to your enemy that maybe just going first and surprising you is the best chance to win. /11

Now, if you're trying to deter just North Korea when it has just one missile, you could argue that "we can shoot down your only asset" is a deterrent. Of course, that presumes that all your other attempts at deterrence have now failed and that's all you've got. /12

Mostly, you're telling a small missile power like NK: "Hurry up and neutralize this small defense by building a lot of missiles and decoys and dummy missiles and other things that we don't do during tests." Meanwhile, you're handing RU and PRC easy propaganda wins. /13

I argue that if you want to deter peer nuclear powers, adopt minimum deterrence and no first use and be clear that nukes exist to deter the use of nukes as existential threats to the United States. For small powers, start building what matters: Conventional forces. /14

"How to deter small nuclear powers" is a separate issue itself, but missile defenses, like nuclear weapons themselves, are a Cold War-era crutch meant to spackle over a strategic hole in our thinking. We should have re-thought this 20 years ago. /15x