This is Part 1 of a multipart series on why the United States wanted to install a THAAD missile defense system in South Korea in 2016, why Seoul agreed, and why China opposed it through economic coercion and other pressure.

Finding the extra bandwidth to think about something that happened last month can be a challenge, let along eight years ago. But if like me, you’re trying to figure out where Greater East Asia is headed, it’s worth spending a moment to reflect on the 2016 South Korean THAAD episode.

Beijing’s decision to whack South Korea with economic coercion and other pressure following United States Forces Korea’s installation of a THAAD missile defense system profoundly transformed the geopolitical landscape. To start with, it stomped on Korean public opinion toward China:

And here’s a different survey showing something similar:

Before he was South Korea’s ambassador to China, Jae Ho Chung was a professor at Seoul National University. This is from his 2006 book, Between Ally and Partner:

It’s now 18 years later, so still a couple of years short of Prof. Chung’s deadline. But clearly, increased contact with the “real China,” with the THAAD event a standout example, has indeed soured South Koreans perceptions toward their giant neighbor.

And let’s not forget, there’s also been a congruent re-enchantment with the US. Opinion polls showed approval for THAAD increased along with China’s pressure campaign:

At least in terms of public opinion, South Korea is now one of the most pro-US/anti-China countries in the world:

This outcome would've completely surprised everyone back in 2006, not only Prof. Chung. The THAAD incident—let’s call it K-THAAD—appears to have been an important factor.

It also paved the way for a new heights in cooperation with Japan under current president Yoon Suk Yeol. This was coming off a low base, to be sure. But still, it was a remarkable shift.

As Prof. Chung noted, attitudes toward China and the US are connected. This is also true with North Korea and Japan. Negative attitudes toward China and North Korea make room for Japan’s numbers to rise, and vice versa:

And here’s one more showing emotions ranging from very cold (0) to very hot (100):

Getting literal beers with Japan is a real political liability in South Korea, and there is a good chance that the current cooperation will break down after the next South Korean election. But still, Beijing can take some credit for the shift.

As I noted in The Diplomat, “A minor theme of Yoon’s presidential campaign was that Moon had been too supine toward Beijing in relation to THAAD and other issues.” And Beijing helped create what I previously described as the “national vibe shift” that facilitated Yoon’s outreach to Japan.

Moving away from Korea, we can also see that the episode boosted the case for China “de-risking” around the globe. As the Australian Strategic Policy Institute put it, “The wide-ranging and severe coercive effort targeting the ROK presented an important learning opportunity for Seoul and other states.”

A universe in which K-THAAD happened is probably quite different from one in which it did not. Below is list of important trends that, while overdetermined, can be chalked up to K-THAAD to some degree:

• South Korea is more wary of China yet less deferential (on net)

• The world is more wary of China yet less deferential (on net)

• China’s nuclear and other missile capacity has become more formidable in absolute and relative terms

• North Korea’s nuclear and other missile capacity has become more formidable in absolute and relative terms

• US forces in Korea and Japan, as well as other targets in Korea, Japan and surrounding seas are better protected in absolute terms (though probably not on net)

• US-Korea alliance strengthened

• Korea-Japan cooperation increased

• China somewhat more cautious about applying economic coercion

• China facing increasing containment yet less “contained” (pace of aggrandizement has slowed but not culminated)

The social creatures that we are, we are not satisfied with simply knowing what happened, we want to know why. And when it comes to other humans, that means we want to know their motivations. This lets us assign blame. It also helps make predictions about what those willful agents will do next. And knowing what China, South Korea and the US wanted to achieve helps us frame the trends I bullet-pointed above, and informs our response.

Assigning motivations to K-THAAD happened from the very beginning, and it will continue to happen. So it’s important to get the best understanding of those motives that we can.

So why did it happen again?

Given how impactful this episode has proven to be, you’d think there’d be a general consensus on why it all went down. But you’d be wrong.

Trying to infer the motivations from its main effects doesn’t help much. Unless you want to believe that “Xi the Accelerator-in-Chief” 总加速师 meme or think that Xi is a CIA asset, it would be odd to think he wanted to drive South Korea into the arms of the US and Japan, and to bolster the formation of an anti-China balancing coalition.

At the same time, it doesn’t look like a master stroke on the part of the US. The threat from both China’s and North Korea’s nuclear and conventional missiles has increased. At least as far as China’s arsenal is concerned, this probably overwhelms any marginal benefit brought by K-THAAD. US forces in the region and its allies and partners are probably more exposed now than in 2016.

Another way to say it is that China’s reaction added more to efforts to push back against China’s power than the missile defense system itself. And while that may contribute to a successful balancing of China’s power in the future, it doesn’t solve the problem that China’s military power relative to the US and its partners in terms of a war of attrition is now greater than it has ever been.

The situation is so serious that it would not be crazy for Xi and North Korea’s Kim Jong Un to think that together they could defeat the US in a war. How we should feel about that comes back again to motives. So let’s get on with it.

Let’s assign a number for each motive!

We want to know what motivated the US’s and South Korea’s decisions to install K-THAAD. And, we want to know what motivated China to oppose it through economic coercion and other pressure.

Knowing that is tricky to say the least. The decisions around K-THAAD are national secrets, and the relevant documents are not available to the public. Even if we could read them, and even if the leaders involved were willing to give candid interviews, we would still not know in an absolute sense. Don’t get me wrong, Xi Jinping sitting down for a K-THAAD Netflix docuseries would be amazing. But those decisions were collective ones taken within an interactive domestic/international context across time. The rationalizations in their heads, or what bureaucrats put down in a top secret memo, are not necessarily the motives in the sense that we are interested in. They would help immensely. They would be crucial bits of information. But they would still be part of a larger picture.

That big picture is one of assigning relative weights to motives. Each weight is our best guess based on the information we have.

There are many ways we can decide on something under conditions of uncertainty. But some are better than others, and one of the best is Bayesian reasoning. It is so good that our perceptual system is build like that.

I notice a rustle behind a bush. Is it a tiger? First, I check the probability against my prior expectation. This is a residential area in Seoul. The last Korean tiger was shot more than a century ago. There being a tiger behind that bush is extremely unlikely, but it’s not completely impossible. There are zoos in Seoul of course. But then a flash of orange and black. How likely am I to see such a flash if it is a tiger compared to being some other thing? A tiger is now seeming more probable, but I’m still not panicking. Maybe it is a poodle in a tiger suit? Maybe it is that mushroom jeongol I had for lunch? I look closer, and see what clearly appears to be…

You get the idea. I may never know for sure if it is a tiger or not. But my best guess gives me the information I need to act: ignore it, pause to gather more info, or run.

Understanding the motives that drove K-THAAD is similar. We have lots of information, with new information coming in. We want to make our best guess about what they are so we can act.

But we don’t want to run a true Bayesian algorithm. That would be too slow. And boring. I would need to put down my priors for each motive, then start reading and updating as I go. Or, I could do all of my reading, form my priors based on that, then wait for the world to provide new information to update me. Also, it would require math.

Instead, we will do what I’m going to call quasi-Bayesian reasoning. I will assign a prior weight for each motive based on what it would be if the US, South Korea and China were rational actors. Then, we will update that based on the information we have to arrive at a final weight in an informal way.

Final is the wrong word here as this is an iterative process. None of my numbers are conclusive. Rather, it is a process of continual updating until we converge on the truth.

Of course, some new information may arise that is compelling yet very different to the information we have already synthesized as the weighting. But that is not likely, and at any rate, we need to make judgements now. So while not perfect, these numbers are intended to be the best possible. If they are not, show me how they are wrong so I can improve them.

Ideally, I would remain open to all hypotheses. I would determine a rational prior for all of them and update them all simultaneously with each piece of information. But that is far beyond my capacity. Instead, let’s take a short cut based on my understanding of the motives that are most relevant.

The resulting most parsimonious list of motives that apply to the three countries involved looks like this:

H1 negate/enhance China’s nuclear second-strike

H2 advantage in regional coercion/conflict

H3 South Korean alignment (other than H1, H2, H4)

H4 suppress/support North Korea

And the result?

How we get there will be laid out in subsequent posts.