Kim Jong-un, North Korea’s brash young leader, traveled with his wife and senior aides. They were shown being welcomed on a red carpet with a military band in the Great Hall of the People in Beijing. According to the Chinese, he held long and productive meetings with China’s president, Xi Jinping. He also attended a theatrical performance and toured the Chinese Academy of Sciences. The most important thing was that Kim said that he was open to a dialogue with the United States — even a summit with Donald Trump. Xi responded that he would return the favor and visit North Korea’s capital, Pyongyang. Chinese officials also later called the White House and briefed them on the meeting. He was making his first trip abroad since taking power in 2011. It was his first time meeting another head of state. What made to visit all the more extraordinary was that it came after a period of great tensions with China and the rest of the world over North Korea’s nuclear weapons program. Those tensions were not at all on display in the photos and videos that were released by the Chinese and North Korean state media on Wednesday. There was a flurry of speculation here at the beginning of the week when a train made its way into the Chinese capital. This is not an ordinary train, but an armored and specially outfitted one: 21 cars with tinted windows. It was like ones used by previous North Korean leaders who visited China. But neither the Chinese nor the North Koreans would confirm anything about it. It was only 36 hours later that we learned who was inside. It is not at all clear if this is a diplomatic breakthrough or not. But with meetings planned in the coming weeks with South Korea’s president, possibly with Mr. Trump, it could be a period of intense diplomacy after months of threats and counter-threats.