Asian allies are already worried about what the Bolton appointment could mean for them:

After months of painstakingly building a relationship with the departing national security adviser, Lt. Gen. H.R. McMaster, South Korean officials now have to manage their “very bad chemistry” with Mr. Bolton, “who is all about sticks,” said Lee Byong-chul, senior fellow at the Institute for Peace and Cooperation in Seoul.

“We will have to see if Bolton opens his mouth and launches his verbal attacks against the North,” Mr. Lee said. “That will give North Korea an excuse to step away from its summit proposal. The Trump-Bolton team then will ramp up pressure. And we will hear more talk about a pre-emptive strike and see tensions rising again on the Korean Peninsula.”

Bolton almost certainly will use incendiary rhetoric about North Korea, but the far bigger problem is that he thinks diplomacy with Pyongyang is useless and will seek to use any meeting with Kim as a pretext for undermining it. He has said that he thinks the purpose of any meeting with Kim is to deliver an ultimatum to him:

Bolton said Trump had short-circuited North Korea’s plan of obtaining the capability to strike the United States with a nuclear weapon and then stretching out negotiations for months that could distract the American government before making an official announcement about achieving the capability. Bolton envisioned the meeting between Trump and Kim as an opportunity to deliver a threat of military action:

“I think this session between the two leaders could well be a fairly brief session where Trump says, ‘Tell me you have begun total denuclearization, because we’re not going to have protracted negotiations, you can tell me right now or we’ll start thinking of something else.’ ”

Bolton dubs this approach “diplomatic shock and awe,” which should be a warning all by itself. Combined with the Trump administration’s past inflexibility on North Korea, that could very well be what happens. Like other hard-liners, Bolton thinks the value of meeting with an adversary is limited to demanding his surrender. When North Korea doesn’t agree to the unrealistic demand, that could set the stage for launching a preventive war that Bolton has already been advocating for a long time:

There are also risks if the talks lead nowhere, because critics of engagement will use that as an opportunity to say that diplomacy has failed.

“I am particularly worried that if the Trump-Kim summit fails, Bolton will take that as proof that we must hit North Korea,” said [Robert] Kelly.

This is why having a high-profile summit with North Korea without doing any of the normal preparation and work that usually goes into these things is so worrisome. Even without Bolton, the summit seemed unlikely to yield any desirable results, and with him it is even less likely. There are always hard-liners eager to seize on the supposed “failure” of diplomacy, and so it is an invitation to serious trouble when an administration merely pays lip service to talking with an adversary without having any serious interest in finding a compromise solution. In this case, one of those hard-liners will be inside the White House briefing the president every day.