China, as North Korea’s strongest ally and biggest trade partner, is a critical element of any possible solution to the North Korea nuclear crisis, which took yet another dangerous turn on Monday night when North Korea launched a ballistic missile over Japan. But so far China appears unable — or unwilling — to curb North Korea’s nuclear ambitions.

To get a better sense of the dynamics at play in this relationship, I spoke with Ely Ratner, an expert on US-China relations and East Asia policy at the Council on Foreign Relations, a nonpartisan think tank in Washington, DC.

I asked Ratner if the stand-off between the US and North Korea is as much a test for China as it is for Trump, and if he believes China is likely to get involved militarily to defend North Korea if a war breaks out in that country.

He told me that China would prefer that North Korea stop testing missiles and start negotiating with the US but "there's no clear path towards that right now." As a result, China has few good options and its primary concern, increasingly, is preventing war. But should a war erupt in North Korea, China would likely have no choice but to intervene, and the consequences of that would be both grave and unpredictable.

You can read our lightly edited conversation below.

Sean Illing

Do you think China’s position on North Korea has shifted one way or the other because of Trump’s, shall we say, unpredictable behavior?

Ely Ratner

In short, the answer is yes. China's position on North Korea has been shifting, but that's been happening over the past couple of years; it's not just over the past six months. So we may have seen some acceleration of that, but it's a continuation of what we've been seeing for the past couple of years, where North Korea has increasingly become a strategic liability for China and debates within Beijing around the North Korea issue are hotly debated.

So there's been an ongoing evolution of policy within Beijing and they've been willing to put more pressure on North Korea. I think the degree to which the Trump administration has signaled that it's willing to exacerbate China's concerns in the absence of a solution has certainly gotten their attention.

Sean Illing

Is the current stand-off between the US and North Korea as much a test for China as it is for Trump?

Ely Ratner

It depends what you mean by "test." It's certainly a difficult situation for China because their preference would be for North Korea to cool down and stop testing missiles and enter into some kind of negotiation track with the US, but it just doesn't appear that the North Koreans are prepared to do that.

So what we're seeing in response is the Trump administration identifying this as a top-tier national security problem where neither side sees off-ramps. And what China wants more than anything is for both sides to deescalate, but there's no clear path towards that right now.

Sean Illing

Is North Korea’s continued testing of missiles a defiance of China as much as it is the US?

Ely Ratner

Absolutely. There's no question about that.

Sean Illing

So what’s North Korea’s calculus here? Are they convinced they won’t pay a price for this defiance?

Ely Ratner

It's always hard to understand what the North Koreans are thinking, but I think part of it is that, yes, they probably think that because China is so concerned about regime collapse in North Korea, they won't be willing to go all the way to destabilize the [Kim Jong Un] regime.

But at the same time, it's important to note that part of what North Korea has been doing over the past couple of years is trying to improve their capabilities. We often use the word "provocation" to describe North Korea's missile tests, and in a sense they are, but the regime is legitimately trying to improve its missile capabilities and not necessarily focused on defying China or the US or the international community.

Sean Illing

What is the aim of China’s policy toward North Korea? Stability? The preservation of the status quo? A strong buffer between it and the democratic South?

Ely Ratner

It's all of those things. Stability on the Korean Peninsula has always been prioritized over denuclearization. Even if denuclearization is in China's interest, it still ranks stability above all else.

The problem is that they don't see a viable path to get there that doesn't create unacceptable risks of instability. So China may want North Korea not to have nuclear weapons, but at the same time there are a whole bunch of negative repercussions for China if it applies too much pressure. For instance, North Korea could turn on China and China could become a direct enemy of North Korea, so instead of North Korea threatening to shoot missiles at Guam it could be threatening to test them in a way that would endanger China.

There are also immediate concerns about Chinese pressure in conjunction with the international community precipitating a crisis of regime collapse in North Korea — and with it a massive flood of refugees crossing the border, loose nuclear materials, and the economic costs of instability on the peninsula. And then there are concerns about the ways in which a post-Kim, unified democratic Korea could become a strategic problem for China.

Sean Illing

China has a lot of leverage over North Korea, being its biggest trading partner and main source of food and energy. Why are they struggling to curb the regime’s behavior?

Ely Ratner

They've lost political leverage in part because the North Korean regime has killed some of the interlocutors with China and so they don't have a real healthy political channel right now. China does have enormous economic leverage over North Korea, but again they're unwilling to use it for fear of destabilizing the North Korean regime. So yes, they have leverage, but they're going to be reluctant to use it.

Sean Illing

China has little love for Kim Jong Un’s regime, but it obviously tolerates it for economic and geopolitical reasons. What would it take for China to decide that the costs of the relationship outweigh the benefits?

Ely Ratner

That's the debate they're having right now. The costs are mounting for them — including enhanced US military capabilities in the region; declining relations with surrounding countries; countries like Japan and South Korea developing their own missile capabilities; increased sanctions putting pressure on Chinese firms and banks; and of course the fear that a full-scale conflict could erupt in North Korea.

So I think to the degree that all of those factors are becoming more extreme, we are reaching the point at which China is willing to do more much more than it has in the past.

Sean Illing

Under what conditions might China get involved militarily to defend North Korea?

Ely Ratner

I think the likelihood of China defending North Korea for the sake of defending North Korea is limited, but I think there's a very high likelihood that in the event of instability on the peninsula, the Chinese military would intervene into North Korea. At the very least, they would want to create a buffer zone and protect their border. But they’d also likely go even deeper into North Korea in order to deal with loose weapons of mass destruction. So I wouldn't think of it as China defending North Korea so much as China defending China's interests within North Korea.

It's a great question, though, because what that means is that there's a lot of uncertainty about what military action on the Korean Peninsula could produce in terms of Chinese intervention, US intervention, and obviously South Korean intervention. And that's why it's so important that, as all this bluster and crisis moves forward, we're having quiet conversations with the Chinese to try to develop at least some shared understanding about how we would communicate and what our expectations would be in the event of war or instability.

So when we talk about diplomacy, we should be talking about how to resolve the North Korean nuclear problem, but we also need to be engaging in diplomacy with the Chinese and the South Koreans and the Japanese about how we might all react in order to help avoid a larger, unintended war.