Were there different phases of this anti-corruption drive, and did its contours reveal something about Xi’s political priorities?

At first, Xi focussed very heavily on the security services—the source of hard power on the civilian side. This allowed Xi to rip apart networks of people in the security services who didn’t necessarily support him, then put his own people in and build up that support. If you’re the dictator and you’re trying to insure that your personal position is secure, you need to consolidate and control the sources of hard power. And the civilian side was really the richest target and the easiest target for him, easier than going after the P.L.A. The P.L.A. has been successful at resisting lots of reforms and cleanups over the years. So he targeted the civilian side first, and then he started working through the P.L.A.

When did that start with the P.L.A.?

The real turning point that signalled that something big was going to happen was the fall of 2014. Back in 1929, Mao had convened the Gutian Conference, which was really about the C.C.P. taking control of the military and really about Mao consolidating his power. Xi effectively reënacted this in 2014, summoning all the top generals to Gutian, and it was clear from the messaging that came out of that meeting, and the steps Xi took afterward, that this was the beginning of this massive anti-corruption campaign inside the P.L.A. I think this had multiple objectives. There was real corruption. There was a massive problem in the P.L.A. where, if you wanted to get promoted to certain levels, you had to actually buy that promotion. So various people would put money up because they figured once this person got promoted, they could get a return, since it would open up all these graft opportunities. It was almost like they were angel-investing in a P.L.A. officer.

There was also the question for Xi was how to unravel these networks and put your own people in, so that you ultimately have control over the P.L.A., and it becomes the kind of fighting force you want.

Does the purging of Zhang Youxia make sense within this strategy, or does it seem like something new?

Zhang was promoted and thrived during the incredibly corrupt era of Hu’s leadership. He oversaw, for a period of time, the P.L.A.’s equipment department and its weapons-development and -acquisition programs, which, given how much the P.L.A.’s budget has increased over the past several decades, had massive graft opportunities. And, since that Gutian meeting, the C.C.P. has been rooting through the top ranks of the P.L.A. Now, the Central Military Commission has been reduced from seven members to Xi and one vice-chairman: Zhang Shengmin.

But why now, and why so quickly? That is something that I don’t have a great answer for. And I have not found anybody who has a great answer. Some people argue that, in order to make an accusation like this, you have to work up the vine, and you have to build cases, which becomes harder and harder the more senior they are. There are rumors that Zhang was building a putsch against Xi. But I think that’s bullshit, and ultimately we really don’t know. It is such a black box.

One theory behind Xi’s military purges you did not bring up was that he wants people who are in line with his foreign-policy priorities.

I talked about how he needed to clean out corruption because he wanted to build a professional fighting force. That is absolutely one of the reasons. It’s about the combination of control over the P.L.A. and insuring the P.L.A. leadership has the right political standing or political positioning, but it is also about having an actually competent P.L.A. that has good weapons, and can fight. The leadership is constantly talking about fighting and winning. Xi’s stated goals for the P.L.A. are all about actually being able to fight and win wars and becoming a world-class army.

Sure, but any leader of any country, democratic, nondemocratic, whatever else, is going to want a military that’s competent. But you may also want a military leadership explicitly aligned with your foreign-policy priorities, whatever those may be. And those strike me as different things.

I think what you’re getting at is the speculation out there that perhaps this latest round of purges was triggered by the fact that Zhang Youxia was not aligned with Xi on Taiwan, for example, and that there was some sort of discord between what Xi thought the P.L.A. should do and what the generals wanted. It’s possible, but I am skeptical of that because I think that the way the system is structured, it would be pretty shocking if the most senior generals had been really pushing back on Xi around that. It’s possible, but we just don’t know, and that’s the problem.

Do we know what happens to high-ranking figures who are purged?

On the civilian side, they’ll usually have a trial, and then it’ll be announced that they’re getting sentenced for some range of years, or for life. Rarely do senior civilian officials get executed. It has happened to some of the people in the financial system who were purged, but in general, they get sent off to a pretty comfortable prison life at a sort of Club Fed-type facility outside of Beijing. On the military side, we don’t know.

There has been a lot of concern about how President Trump has alienated NATO allies in recent months, leading to questions about how this may reshape American foreign policy in some fundamental way. Do you have any sense of whether the Chinese government thinks the Trump era could dramatically reshape international relations? And could that be to China’s advantage?

I think if you go back to what Xi has been saying for years, he’s been talking about how we are in an era where there are changes in the global landscape unseen in a century, and the Trump Administration’s recent moves just reinforce what he’s been saying about how the world is changing. So the C.C.P. absolutely does think that the world is changing.

I think it’s a mixed bag for them. On the one hand, it’s creating a lot of opportunities for their external propaganda approach, which for many years has been about weakening the U.S. position in the global order as much as they can. We are now helping them with that cause in a lot of ways, more than maybe some previous Administrations did. But, at the same time, the C.C.P. also benefitted a lot from the U.S.-led order. They are, I think, concerned about some sort of sudden collapse into real chaos. And so I think they would prefer to see a managed decline of the order, where they can more thoughtfully find ways to exploit it, which I think they’ve already been doing over the last decade or so.