Thread by @james_acton32

<THREAD>Thoughts on the successful test of an SM-3 IIA interceptor against an ICBM.

BLUF: For technical & operational reasons, SM-IIAs aren't much use for homeland defense, especially against Russia or China.

Politically this test is a BIG deal. (1/n)

https://twitter.com/LucasFoxNews/status/1328620620100329472?s=20

The idea of using SM-3 IIAs against ICBMs isn't new, though interest has ebbed and flowed over the last decade. By demonstrating at least some capacity in a test, however, this idea is no longer just theoretical.

But is it practical? (2/n)

Given the locations of the target launch (Kwajelain Atoll) and the interceptor (northeast of Hawaii), it's clear that the target missile was intercepted on its way down.

See this helpful picture dug up by my colleagues, @nktpnd. (3/n)

https://twitter.com/nktpnd/status/1328698562532749313?s=20

Intercepting an ICBM shortly before reentry has two implications for homeland defense.

First, a single interceptor can only cover a relatively small area--like a cornerback at football. So, you need lots of interceptors to protect a wide area. (4/n)

Second, interceptors would need to be located near the homeland.

Now, the U.S. has lots of Aegis capable ships that could be loaded with SM-3 IIAs. So, *in theory*, it could park them around the coasts and hence supplement its missile defenses. (5/n)

But, it's hardly practical because those ships have, you know, other missions. In fact, especially in a war against Russia or China--when nuclear use could come into play--those ships would be need to be forward deployed. Not least to protect carriers. (6/n)

There simply wouldn't be time to redeploy the ships around the United States if nuclear use by Russia or China began to seem like a real possibility.

Ship-based homeland defense would be somewhat more practical in a war against North Korea, but only somewhat. (7/n)

And, to be sure, the Trump administration has never claimed that ship-based defense would be practical against Russia or China! The Missile Defense Review explicitly singles out "rogue states" here.

media.defense.gov/2019/Jan/17/20…, page 55.

There's also technical challenges associated with the use of SM-3 IIAs against real ICBMs.

The test appears to have been conducted over a range of 5,000-6,000 km. Real ICBMs have significantly longer ranges and hence travel at greater speeds, complicating intercept. (9/n)

Moreover, real ICBMs have anti-BMD countermeasures. Given that the test readout doesn't mention countermeasures, it's a safe bet that the target missile didn't have them. (10/n)

Now, to be sure, there are other concepts of operations. The U.S. could install land-based SM-3 IIAs around the homeland. That'd be very expensive and you still have the problem of countermeasures. (11/n)

Which leaves you with forward-deployed land-based SM-3 IIAs. Otherwise known as Aegis Ashore! Indeed, there are already plans to install these interceptors at the Polish site. (12/n)

Now, most analysis to date suggests that *forward-deployed* SM-3 IIAs lack the speed to catch Russian ICBMs. And today's test doesn't undermine that analysis. (Though if future testing does, that'd be highly significant.) (13/n)

SO... all in all, I don't think today's test really threatens Russia's or China's nuclear deterrents.

BUT there's no question they--Russia especially--won't see it that way. (14/n)

Russia will see this test as vindication that its concerns about the European Aegis Ashore system have always been correct. And, given those concerns are fundamentally about the long term future of BMD technology, I think they are genuine. (15/n)

In short, this test is likely to continue stimulating Chinese and Russian efforts to bolster their nuclear forces and make arms control to hinder the burgeoning arms race more difficult. (16/16)

Please see this addendum. (17/17)

https://twitter.com/james_acton32/status/1328748192079425537?s=20