Demo­cratic People’s Re­pub­lic of Korea leader Kim Jong Un’s visit to Bei­jing to meet with Chi­nese Pres­i­dent Xi Jin­ping, in ad­vance of his up­com­ing meet­ings with South Korean Pres­i­dent Moon Jae-in in April and United States Pres­i­dent Don­ald J. Trump in May, ought to be a cau­tion to Mr. Trump and his new sec­re­tary of state: Step care­fully and do your home­work.

The visit cast the North Korean leader as rea­son­able and will­ing to ne­go­ti­ate, in­clud­ing, per­haps in stages, de­nu­cle­ar­iza­tion.

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By hold­ing the meet­ing the Chi­nese gov­ern­ment in­serted it­self into the peace pro­cess. The play­ers would not sim­ply in­clude the North Koreans, the South Koreans, and the Amer­i­cans. The Chi­nese would have some­thing to say about the Korean pen­insula.

But the Korean leader also went to China as an equal, not a sup­pli­cant. As sev­eral ex­perts have noted, there is no ev­i­dence that he asked China’s per­mis­sion to hold the sum­mit with Mr. Trump.

It is im­por­tant to re­mem­ber that the Korean pen­in­sula is geo­graph­i­cally a rel­a­tively small off­shoot of China, with a short bor­der with Rus­sia, ex­tend­ing be­tween the Yel­low Sea and the Sea of Japan. The pop­u­la­tion of the two Koreas is 75 mil­lion; that of China, 1.4 bil­lion. In stra­te­gic terms, the word to de­scribe Korea’s sit­u­a­tion is “vul­ner­a­ble.”

Even more rel­e­vant to Mr. Kim’s visit to Bei­jing, his first for­eign trip since be­com­ing North Korea’s leader, is the se­quence of events in the Korean War. His grand­father Kim Il Sung’s army in­vaded South Korea in 1950 and drove down to the tip of South Korea be­fore U.S. forces were or­ga­nized enough to re­pel them. Amer­i­can forces then pushed the North Koreans nearly to the Yalu River, on the bor­der with China. Then the Chi­nese People’s Lib­er­a­tion Army in­ter­vened and pushed the U.S. forces back in bit­ter fight­ing to the 38th par­al­lel, pretty much where the war started, and where ev­ery­one has re­mained.

If any­one is think­ing about the United States fight­ing an­other war in Korea, he should read care­fully a his­tory of the 1950-1953 one. The Korean pen­in­sula is a harsh set­ting for a war.

In any case, with this visit a his­toric and per­sonal re­la­tion­ship has been re­paired and re­newed, and the Chi­nese have as­serted themsleves, but not, let us note, at U.S. bid­ding — at their own and North Korea’s.

The Amer­i­can pres­i­dent and his new for­eign pol­icy team must ac­com­plish a very del­i­cate, per­haps im­pos­sible task: They must convince and com­mit Mr. Kim to de­nu­cle­ar­iza­tion, while at the same time giv­ing him am­ple as­sur­ance that he will re­tain a strong na­tional de­fense and am­ple de­ter­rence. It is pos­si­ble that such a com­plex and sen­si­tive for­mu­la­tion will be eas­ier with the Chi­nese in­volved, but only pos­si­ble.

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First Published March 30, 2018, 9:15 p.m.